Khoren Sahagian

Khoren Sahagian
Materials Scientist

Editorial July 2014

Plasma treatments are a permanent and covalent substrate modification.  However many references note diminishing effects of plasma treatments with time.  One generalized conclusion is that the plasma modification is a temporary effect.  This conclusion is not inherently accurate or applicable to all plasma and material systems.  In truth there are many factors that govern the success and longevity of a plasma modification.  Research in plasma lacks harmonization in equipment, setup/configuration, and material selection.  These are key variables in a plasma modification.  Results from one method may not necessarily translate well to another experimental setup or class of material.  For this reason some engineering reviews of gas plasma do more to confound than to elucidate the scientific dialogue within industry.

 

Equipment design is of particular relevance in plasma industry.  This includes but is not limited to the electrode configuration, matching, RF frequency, and equipment geometry.  Many apparatus used in academia boast custom fabricated equipment or custom modification to existing tools.  While their equipment exemplifies engineering capabilities, the research does not always consider all parameters fundamentally related to performance.

 

Plasma chemistry and substrate material should be matched correctly.  Some polymer systems may be either resistant or sensitive to specific plasma chemistry.  It is not enough to report gas, pressure, and power.   A complete characterization should understand the plasma stoichiometry and a hypothesis of the surface interaction.  Furthermore it must be accepted that many polymer systems are mobile, may swell with gas or moisture, or may undergo relaxation mechanisms.  Therefore be careful to consider pairing a material system with appropriate plasma source and plasma chemistry.

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7. July 2014   2:26 pm
Dr. K. L. Mittal, Dr. Robert H. Lacombe

Dr. K. L. Mittal, Dr. Robert H. Lacombe

We continue the INVISIBLE UNIVERSE blog after a delay due to our heavy involvement in the recently completed 9th INTERNATIONAL SYMPOSIUM ON CONTACT ANGLE, WETTABILITY AND ADHESION held at Lehigh University June 16-18, 2014.

Speaking of CONTACT ANGLE phenomena this would be a good time to review this topic as the contact angle method is by far and away the most popular surface analysis method in use today. This technique is of special interest to anyone doing surface modification either by plasma or any other method as it is the simplest and least expensive method for analyzing the impact of any surface treatment whatever. It is fortunate that our office has received a recently published volume on this topic which we will review shortly but first a rudimentary introduction to the concept of contact angle behavior of droplets for the sake of those who may be new to this topic.

Classic definition of the equilibrium contact angle of a drop of liquid on a surface as the balance of three surface tensions.

Fig.(1). Classic definition of the equilibrium contact angle of a drop of liquid on a surface as the balance of three surface tensions.

When a drop of some liquid is placed on a surface it will typically bead up and form a sessile drop as exhibited in Fig.(1). The angle that the edge of the drop makes with the underlying solid is determined by the balance of three surface tensions or surface energies if you prefer. The concept of surface energy has been covered in some detail in the February 2014 issue of this blog so it will not be reviewed here only to point out that surface tensions as measured in Nt/m are dimensionally the same as surface energies (J/m2) which are readily derived from surface tensions by multiplying the numerator and denominator of the units by m (meter) giving Nt-m/m2 or J(joules)/m2.

And now to our book review:

Wetting of Real Surfaces, By Edward Yu. Bormashenko, (Walter de Gruyter GmbH, Berlin/Boston, 2013)

In this rather compact monograph Prof. Bormashenko provides a rather comprehensive account of the theoretical developments in the field of contact angle and wettability of surfaces.

Interestingly, Prof. Bormashenko points out that the field of contact angle and wettability remained rather a backwater endeavor in the field of modern physics from the time of Thomas Young’s pioneering work up to roughly the 1990′s despite the fact that scientific heavyweights such as Einstein, Schrödinger and Bohr devoted a significant portion of their research activity to this topic. Much of this stems from the fact that surfaces presented a rather messy and intractable research topic due to the difficulty in obtaining well defined surfaces free of contamination and other defects. Indeed the eminent theoretical physicist Wolfgang Pauli remarked that “God created matter but the Devil created surfaces”. Thus the solid state physics literature up to about the early 1980′s tended to be dominated by topics such as superconductivity, electronic band structure, phase transitions, semiconductors and similar topics dealing primarily with the bulk behavior of solids.

This all started to change significantly by about the 1980′s being led in large part by the microelectronics industry which was fabricating multilevel thin film structures which were becoming more and more dominated by interfacial surfaces between metals, insulators and semiconductors.  Even by the early 1970′s it was becoming apparent that in order to fabricate devices with higher and higher circuit densities it was critical to understand the nature of the interactions between the various material components at their contact surfaces. This need was supported by advances in microscopy starting with electron microscopy and evolving further to electron tunneling microscopy and finally to the now ubiquitous atomic force microscopy. On top of this a number of surface analysis techniques emerged nearly too numerous to mention the most popular of which being X-ray Photoelectron Spectroscopy (XPS also called ESCA Electron Spectroscopy for Chemical Analysis).

The need for understanding surface properties was of course not limited to the microelectronics industry. The entire coatings industry needed to understand the wetting properties of various paints and inks and the biotechnology industry dealing with medical implants needed to understand how the surfaces of their devices would interact in the in vivo environment. The contact angle technique thus started to emerge as a low cost and highly sensitive method for exploring the wetting behavior of surfaces.

A critical juncture of sorts was achieved with the work of Barthlott and Neinhuis in 1997[1] who first studied the extreme hydrophobicity of the lotus leaf and its effect in removing all manner detritus from the leaf’s surface. This work lead to a literal explosion of work on the superhydrophobic effect and a variety of applications to self cleaning surfaces and other highly innovative technologies.

Getting back to Prof. Bormashenko’s volume a brief look at the table of contents reveals a rather wide range of topics:

  1. What is surface tension
  2. Wetting of ideal surfaces
  3. Contact angle hysteresis
  4. Dynamics of wetting
  5. Wetting of rough and chemically heterogeneous surfaces: the Wenzel and Cassie models
  6. Superhydrophobicity, superhydrophilicity and the rose petal effect
  7. Wetting transitions on rough surfaces
  8. Electrowetting and wetting in the presence of external fields
  9. Nonstick droplets

There is clearly not enough space here to cover all of the above topics in any detail so we will focus on chapter 6 dealing with super hydrophobic and hydrophilic phenomena which, as Prof. Bormashenko points out, are among the currently most actively researched topics in the contact angle field.

Schematic illustrating the difference between a truly superhydrophobic surface and one exhibiting only a high contact angle

Fig.(2). Schematic illustrating the difference between a truly superhydrophobic surface and one exhibiting only a high contact angle

Interestingly, the author points out that exhibiting a high contact angle is not sufficient to define a state of superhydrophobicity as might casually be assumed. In addition the contacting water drop must also exhibit low contact angle hysterisis.  That is the advancing and receding contact angles must be approximately the same. This property is required for the so called “lotus leaf” effect where water drops not only form with a high contact angle but also roll very easily off the leaf carrying any collected debris with them as shown in Fig.(2).

The counter example is the “rose petal” effect reported on by Jiang and co-workers.[2]  These investigators looked at water droplets on rose petals which also form very high contact angles but unlike the lotus leaf case these drops also exhibit a very strong hysterisis. An immediate consequence is that these drops do not roll even when held at a steep angle as also shown in Fig.(2).

Example of a hierarchical relief morphology.

Fig.(3). Example of a hierarchical relief morphology.

 A further subtle point brought out in the chapter is the fact that the substrate material does not have to be highly hydrophobic in order to exhibit the superhydrophobic effect. The lotus leaf material is in fact hydrophilic. What gives rise to the superhydrophobic behavior is the hierarchical relief morphology of the surface. An example of such a structure would be the fractal Koch curve shown schematically in Fig.(3). The author goes on to analyze the wetting of these highly variegated surfaces in terms of the Wenzel and Cassie models covered in chapter 5.

In all this volume can be highly recommended to anyone interested in coming up to date on the latest theoretical developments in the rapidly expanding field of contact angle phenomena.

The author invites any inquiries or comments on this article.


 [1] “Purity of the Sacred Lotus or Escape from Contamination in Biological Surfaces”, W. Barthlott and C. Neinhuis, Planta, 202, 1 (1997).<
[2] “Petal effect: A superhydrophobic state with high adhesive force”, L. Feng, Y. Zhang, J. Xi, Y. Zhu, N. Wang, F. Xia and L. Jiang, Langmuir, 24, 4114 (2008).
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Mikki Larner

Mikki Larner
Vice President Sales & Marketing
Belmont, CA

Editorial May 2014

I sell gas plasma technology.

This can be confusing, as there are several types of plasmas, both naturally occurring (such as the Northern Lights, lightning, and stars) and human-made (such as those used in neon signs, fluorescent lights, and plasma televisions).

From the examples above, it’s clear that plasma generates both light and energy. Plasma can also be used to modify – or, more specifically, molecularly re-engineer – other materials.

My company sells plasma technologies and processes for modifying a myriad of materials. Typically, the application is a surface cleaning and activation – either to prepare plastic or metal for a subsequent coating or bonding step — or thin film coatings that may be used to change the barrier or coefficient properties of a surface.

There are many different ways to manufacture human-made plasma.
We use primarily atmospheric and low-pressure plasma technologies in our work. There are a number of benefits to the low-pressure approach:

1.   For starters, the working environment is a primary plasma. In a primary plasma, there is a greater mean free path of the particles before a collision.

This sustained energy is ideal for modifying the interstices of porous media (such as a non-woven or sintered polymer), or for use inside complex nano-scale vias or channels. With atmospheric processes, on the other hand, the mean free path is very short, so the treatment area is limited.

2.   Low-pressure plasma offers chemistry versatility. Many different gases and vapors can be used, safely and economically. Low-pressure plasma is often used as a replacement technology for wet chemistry processes, providing greater control, lower costs, and lower risk of workplace exposures that could lead to accident or injury.

Additionally, unlike many wet chemistry processes, rinsing and curing is not required with a low-pressure process. This means a much shorter processing time, minutes versus hours in some cases.

In atmospheric processes, use of these chemistries may be dangerous and quantities required to generate the plasma may not be economical. This is one of the reasons that our Openair® technology uses just air. It’s incredibly cheap, readily available, and great for many industrial high-speed surface preparation processes.

3.   When using a low-pressure technique, multiple steps may be run in a single process. A part may be exposed to a cleaning gas chemistry (to remove contaminants from a surface) as well as an activation or coating process in a single run. It is not unusual for a single plasma process to replace two to three manual steps, eliminating overhead costs associated with transporting product, labor and materials.

4.   Another advantage is that the low-pressure process provides an extremely controlled environment. The process is conducted in a vacuum chamber with exacting control of gas flow, time, and power. Variations in the day-to-day environment are removed, and the precise process is readily reproducible. Additionally, cleanliness is assured, whether the process is practiced in a clean room or on an industrial manufacturing floor.

5.   Low-pressure technology allows for permanent, stable results. This means that a large batch of parts may be treated and stored prior to use. Or, alternately, parts may be shipped to other manufacturing sites for final assembly.

6.   Our low-temperature process enables treatment of thermally sensitive materials, and the process is free from electrical potential. Therefore, conductive materials may be safely modified.

7.   The total cost of consumables, including energy, gases/liquids, and maintenance parts, is typically less than $5 per hour. Furthermore, there are no additional costs for hazardous waste disposal, as none is created.

8.   The technology offers high-batch throughput:

•   Line speeds, in our standard R2R equipment, are up to 100 fpm with again, no time require for curing or drying steps.
•   Cycle times, during batch processing, range from 60 seconds to 20 minutes. Because a single batch may include hundreds or even thousands of parts, this means that each individual part is treated in mere fractions of a second.

Our job is to accurately evaluate your application and select the most appropriate technology solution for your production goals, be it low-pressure, corona, flame, or Openair. In rare cases, a simple IPA wipe may be all that’s needed to solve your adhesion problems!

Thanks for reading. If you have any questions, I welcome your calls and emails.

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Khoren Sahagian

Khoren Sahagian
Materials Scientist

Editorial May 2014

The current generation of consumers will eventually become displaced by the millennials.  Product development and marketing experts will be learning to cope with the new idiosyncrasies of ‘Generation Y’.  Firstly, many of these individuals do not form strong allegiances to brands.  Second, emotional connections appear to have the greatest dominance over consumer selection.  And finally, there is greater importance in first discovery.  The arising rules are reminiscent of a Japanese candy bar shelf; no consecutive month will display the same colors, cartoons, or shapes.  So what are some potential implications to the consumer manufacturing space?  Some trends are already becoming clear.

GenY Automation: versatile robotic platforms continue to be integral in production implementation.  Some of the new mechanization is more mobile, easily programmable, rapidly deployed, and cross disciplined in many different categories of operation.

GenY Materials: Whether olefin or bio-based, the custom polymer formulation could lose attractiveness.  Engineers could abandon the new design of materials with chemistry for more accessible technologies that alleviate material constraints with processing methods.

GenY Fabrication:  3D printing promises new and extremely custom fabrication that are uninhibited by classical machine tools or setup.  DIY design will empower individuals to create niche products for myriad markets.  And after this revolution 4D printing envisions the self-assembly of structures likened to proteins inside living bodies.

GenY Environment: Future consumers assert a greater demand for re-usability and a low environmental impact.  There is a less tolerance for waste in an ever shrinking planet with finite resource.

The manufacturing plants that are best adapted to the changing landscape will claim the lion’s share of consumer purchase.  The challenges will be non-trivial.  On the inside consumer products will require simple molecules that are biodegradable, easily formed, and bond-able.  And on the outside these products may take on radical forms, become regional fads, and short life-cycle.

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3. April 2014   12:39 pm
Dr. K. L. Mittal, Dr. Robert H. Lacombe

Dr. K. L. Mittal, Dr. Robert H. Lacombe

Is Nylon Hydrophobic, Hydrophilic or Maybe Both?

In a recent posting on LINKEDIN Scott Sabreen Owner-President, The Sabreen Group Inc. Initiated the following discussion:

 

Nylons are inherently difficult to bond because they are hydrophobic, chemically inert and possess poor surface wetting. …

Nylons are hygroscopic and will absorb moisture in excess of 3 percent of its mass of water from the atmosphere. Moisture, in and of itself, creates adhesion problems.  …

 

Hold on, on the face of it the above remarks would seem to be mutually contradictory.  Is nylon hydophobic or hydrophilic?

The resolution of this apparent paradox comes in recognizing that the hydrophobic behavior of nylon is a surface property and the hydrophilic behavior is a bulk property.

Since nylon is an organic polymer it has a relatively low surface energy as do most polymers.  This is a consequence of the surface chemistry and surface physics of polymers and other organics as discussed in the previous edition of this blog.

However the amide groups in the nylon chain attract water and they give rise to the hydrophilic behavior of this material in regard to BULK ABSORPTION of water.  A number of other polymers such as the polyimides also behave in a similar manner.

So in the bulk nylon can behave as a hydrophilic material but on the surface it can exhibit hydrophobic behavior. Just another hidden property of surfaces that make them both tricky and fascinating to study.

The author invites any inquiries or comments.

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3. March 2014   12:35 pm
Dr. K. L. Mittal, Dr. Robert H. Lacombe

Dr. K. L. Mittal, Dr. Robert H. Lacombe

Origin of Surface Energy

In the December 2013 issue of this blog we noted that nearly all the information we commonly acquire about any given surface comes from the radiation reflected from it.  We also noted that of the entire range of the electromagnetic spectrum available that a surface could possibly radiate, we detect only the so called visible spectrum which amounts to barely 2% of what could be emitted. Thus what our eyes alone tell us about what is happening on a given surface is very limited indeed.  Not only that but the type of information is limited to basically the bulk geometry, gross surface morphology and color.

The fact of the matter is that an entire universe of important physical properties remain invisible to our eyes.  The invisible  property we want to explore here in fact cannot be seen even in principle.  This property is what is called the surface energy and to get a picture of it we need the apparatus of thermodynamics.

The whole concept of energy is rather subtle and intricate in general.  In particular it can take many forms including:

  • Electromagnetic energy stored in electric fields
  • Magnetic energy stored in magnetic fields
  • Thermal energy stored in any material at a finite temperature
  • Potential energy of any mass in a gravitational field
  • Kinetic energy of any moving object
  • Relativistic energy of any massive object as given by Einstein’s famous formula E = mc2
  • etc.

However, for our purposes we only need to understand the elastic energy stored in common solids and we can approach this by considering the behavior of a common spring.  Stretch or compress a spring and it will store a certain amount of elastic energy which can be perceived by allowing the spring to return to its equilibrium length. Much the same type of behavior goes on in common solids.  Figure (1) gives a highly idealized but reasonably realistic picture of a solid material viewed at the atomic level.

Schematic diagram of a solid viewed at the atomic level

Fig. 1: Schematic diagram of a solid viewed at the atomic level. To a first approximation the atoms can be thought of as being held together by microscopic springs which account for the elastic properties of the material.

The atoms/molecules in a given solid are held together by atomic and intermolecular forces which arise from the rather complex electromagnetic interactions among the electrons and nuclei which make up the bulk of any material. Fortunately, near equilibrium and for small deformations these interactions behave in a linear fashion very much like the behavior of simple springs.  Thus as Robert Hook pointed out more than a century ago the restoring force tending to bind the atoms together increases in a linear fashion as they tend to separate from one another. Things get quite a bit more complicated at large deformations but that need not concern us here.

Referring to the upper diagram in figure (1) we see that a typical atom in the bulk of our hypothetical solid feels either tensile or compressive loads from all directions and much the same is experienced by all the rest of the atoms in the deep interior of the solid.  However, the situation is quite a bit different for those atoms at or near the surface as shown in the bottom diagram of figure (1).  Looking down into the bulk of the material they see that same forces as the bulk atoms do but now there is no material on top which creates a highly asymmetrical situation.  It is precisely this asymmetry that gives rise to the unique surface tension or surface energy of the solid.

 

A Word on Units

Perhaps one of the most confusing things about surface energies are the units they are expressed in so we take a quick break here to clear up this issue.  Going back to our spring, if we stretch it there arises an immediate force tending to return it to the un stretched length.  Current international convention expresses this force in the standard SI units[1] of newtons. All systems of units are essentially arbitrary but it is nonetheless important to settle on a common standard.  Thus the common SI units for force are the newton with the dyne and the pound also in use but not considered standard by the international community.  The newton is the canonical unit of the international scientific community and the dyne is a scaled down derivative.  The pound is an archaic holdover from the past but is still much in use in commercial transactions especially in the USA.

Focusing on the newton it is formally defined as the force required to accelerate a one kilogram weight resting on a friction free surface one meter per second per second.  That means that the weight increases its speed by one meter per second every second under the action of the applied 1 newton load.  We can also thing of the newton in more intuitive if less rigorous terms as the weight of a common apple at sea level.  Thus if you hold a standard sized apple it imparts a force of close to one newton on your hand.  In terms of pounds the apple weighs slightly more that 1/5 th of a pound and in dynes it weighs about 100,000 dynes.

With the concept of force now rigorously defined we move on to the concept of energy again using our standard apple as a prop. Force times distance is energy in the form or work. Let’s assume that the apple weighs exactly 1 newton.  If we raise the apple from the ground to a height of 1 meter we will have done 1 joules worth of work or putting it a little differently we will have increased the apple’s gravitational energy by one joule.

Getting back to our spring, it stores what is called elastic energy.  As stated above energy is force times distance and the restoring force of a spring is proportional to the extension so the energy stored in an extended spring is proportional to force times distance squared.  These ideas can all be compactly summarized in the following formulas:

F = -k x (Restoring force exerted by a stretched spring) (1)

Where:
F = Force in newtons
x = Displacement in meters
k = Spring constant in newtons/meter

The minus sign in Eq(1) indicates the force is always a restoring force tending to oppose any extension or compression.

The energy stored in the spring is the integral of Eq(1) from 0 to some extension d:

W = IFxdx = -(½)kd2 (Energy stored in stretched spring) (2)

Thus if our spring has a spring constant of 1 newton/meter and we extend it to a distance of 1 meter it will pull back with a force of 1 newton.  Also it will store an energy of ½ joule.

We can think of the spring as having a tension of 1 newton/meter which is just another name for the spring constant.  Turning now to the physical surface of a polymer such as nylon the springs binding the atomic units together would have a tension of about 8×10-25 newtons/meter and this is the basis of the so called surface tension or surface energy of this material.  Now in practice we do not deal with such impossibly small numbers so we need to scale things up a bit.  In the case of our nylon, one square meter of the surface will contain something like 5×1022 molecular bonds among the surface moieties or in terms of our simple spring model 5×1022 springs.  So we take the surface tension of our polymer to be the tension in a single bond times the total number of bonds in a square meter which for our nylon material comes to 40×10-3 newtons/meter.  This is still to awkward so we introduce the milli newton (abbr mN) which is 10-3 newtons so nylon now has a surface tension of 40mN/m (abbreviating the meter as m).  Well why stop here.  We can do a little algebra on the units and say that 40mN/m is the same as 40mN-m/m2 by multiplying numerator and denominator by m.  Now the mN-m we recognize as a mJ or milli joule and so our nylon can be thought of as having a surface tension (aka surface energy) of 40mJ/m2.  And it does not stop here.  Many folks dealing with surface tension measurements would rather not have to deal with the milli prefix and such huge surface areas as a square meter.  It is more convenient to scale down the force unit to dynes (10-5 newtons) and use square centimeters (abbr cm) instead of square meters.  Thus 40mN/m scales down to 40 dynes/cm.

 

Real Solid Surfaces

Now that we have pinned down suitable units for measuring surface energies we can have a closer look at real material surfaces.  The diagrams in figure 1 give a much better depiction of a liquid surface than they do for a solid.  Liquids have a high mobility and can always adjust their configuration to give a uniform surface of minimum surface tension.  Not so with solids.  The surface configuration of a solid depends sensitively on the thermal-mechanical loading conditions under which it was created.  Was the material cooled rapidly or slowly?  What type of loads if any were active during the cooling process?

More realistic depiction of an actual solid surface

Fig. 2: More realistic depiction of an actual solid surface

Figure 2 gives a more realistic depiction of what a typical solid surface looks like.  This figure depicts 4 typical surface flaws that can significantly alter the surface energy of any real solid:

  1. VOIDS: Materials that have been rapidly quenched may not have time to completely condense giving rise to voids which are a source of tensile stress that will alter the surface energy in their vicinity.
  2. INCLUSIONS: No material is 100% pure and contamination species have a strong tendency to migrate toward surfaces where they upset the normal packing and in many cases give rise to a local compressive stress.
  3. GRAIN BOUNDARIES/DISLOCATIONS: Nearly all crystalline and semi crystalline materials are polycrystalline in nature.  That is they are made up of an aggregate of a large number of small crystals all packed together in no particular order.  The boundary where two crystallites meet form what is called a grain boundary.  Further the misalignment of planes within the crystalline give rise to what are called dislocations.  These and other imperfections can give rise to local stress fields where they intersect a surface that again alter the local surface energy.
  4. CONTAMINATION: Of all the surface imperfections contamination layers have the profoundest effect on surface energies.  Real material objects sit around on benches in the lab or other platforms where they are subject to constant bombardment from all the contaminants and gases in a typical atmosphere not the mention the greasy fingers of human handlers.

Needless to say all of these considerations make the accurate measurement of the surface energies of solids a rather tricky business. But enough for now.  We take up this question in the next chapter.

The author invites any inquiries or comments.

 

 


            [1]The International System of Units (abbreviated SI from French: Le Système international d’unités). A bunch of folks got together and formed the General Conference on Weights and Measures, an organization set up by the Convention of the Metre in 1875, which succeeded in bringing together many international organizations to agree not only the definitions of the SI, but also rules on writing and presenting measurements in a standardized manner around the globe.

 

 

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17. February 2014   8:30 am
Dr. K. L. Mittal, Dr. Robert H. Lacombe

Dr. K. L. Mittal, Dr. Robert H. Lacombe

We interrupt the normal flow of this blog on “SURFACES: THE INVISIBLE UNIVERSE”  to present a book review on a volume which should be of keen interest to all working in the field of atmospheric  plasma technology.  The volume in question is:

ATMOSPHERIC PRESSURE PLASMA TREATMENT OF POLYMERS: RELEVANCE TO ADHESION;  Eds. Michael Thomas and K. L. Mittal (WILEY-Scrivener Publishing, 2013)

The volume contains 15 review articles ranging from surface modification with plasma printing  to the deposition of nanosilica coatings on plasma activated polyethylene.  Each article has been produced by leading experts in the respective topics and gives a truly authoritative examination of the subject matter under review.  A quick look at the table of contents reveals the remarkable scope of this volume.

PART 1: FUNDAMENTAL ASPECTS

  1. Combinatorial Plasma-based Surface Modification by Means of Plasma Printing with Gas-carrying Plasma Stamps at Ambient Pressure;  Alena Hinze, Andrew Marchesseault, Stephanus Büttgenbach, Michael Thomas and Claus-Peter Klages
  2. Treatment of Polymer Surfaces with Surface Dielectric Barrier Discharge Plasmas; Marcel Šimor and Yves Creyghton
  3. Selective Surface Modification of Polymeric Materials by Atmospheric-Pressure Plasmas: Selective Substitution Reactions on Polymer Surfaces by Different Plasmas; Norihiro Inagaki
  4. Permanence of Functional Groups at Polyolefin Surfaces Introduced by Dielectric Barrier Discharge Pretreatment in Presence of Aerosols;  R. Mix, J. F. Friedrich and N. Inagaki
  5. Achieving Nano-scale Surface Structure on Wool Fabric by Atmospheric Pressure Plasma Treatment;  C. W. Kan, W. Y. I Tsoi, C. W. M. Yuen, T. M. Choi and T. B. Tang
  6. Deposition of Nanosilica Coatings on Plasma Activated Polyethylene Films;  D. D.  Pappas, A. A. Bujanda, J. A. Orlicki, J. D. Demaree, J. K. Hirvonen, R. E. Jensen and S. H. McKnight
  7. Atmospheric Plasma Treatment of Polymers for Biomedical Applications;  N. Gomathi, A. K. Chanda and S. Neogi

PART 2: ADHESION ENHANCEMENT

  1. Atmospheric Pressure Plasma Polymerization Surface Treatments by Dielectric Barrier Discharge for Enhanced Polymer-polymer and Metal-polymer Adhesion;  Maryline Moreno-Couranjou, Nicolas D. Boscher, David Duday, Rémy Maurau, Elodie Lecoq and Patrick Choquet
  2. Adhesion Improvement by Nitrogen Functionalization of Polymers Using DBD-based Plasma Sources at Ambient Pressure;  Michael Thomas, Marko Eichler, Kristina Lachmann, Jochen Borris, Alena Hinze and Klaus-Peter Klages
  3. Adhesion Improvement of Polypropylene through Aerosol Assisted Plasma Deposition at Atmospheric Pressure;  Marorie Dubreuil, Erik Bongaers and Dirk Vangeneugden
  4. The Effect of Helium-Air, Helium-Water, Helium-Oxygen and Helium-Nitrogen Atmospheric Pressure Plasmas on the Adhesion Strength of Polyethylene;  Victor Rodriguez-Santiago, Andres A. Bujanda, Kenneth E. Strawhecker and Daphne D. Pappas
  5. Atmospheric Plasma Surface Treatment of Styrene-Butadiene Rubber: Study of Adhesion Ageing Effects;  Cátia A. Carreira, Ricardo M. Silva, Vera V. Pinto, Maria José Ferreira, Fernando Sousa, Fernando Silva and Carlos M. Pereira
  6. Atmospheric Plasma Treatment in Extrusion Coating:  Part 1 Surface Wetting and LDPE Adhesion to Paper;  Mikko Tuominen, J. Lavonen, H. Teisala, M. Stepien and J. Kuusipalo
  7. Atmospheric Plasma Treatment in Extrusion Coating:  Part 2 Surface Modification of LDPE and PP Coated Papers;  Mikko Tuominen, J. Lavonen, J. Lahti and J. Kuusipalo
  8. Achieving Enhanced Fracture Toughness of Adhesively Bonded Cured Composite Joint Systems Using Atmospheric Pressure Plasma Treatments;  Amsarani Ramamoorthy, Joseph Mohan, Greg Byrne, Neal Murphy, Alojz  Ivankoviv and Denis P. Dowling
Schematic two dimensional slice through a typical plasma stamp

Schematic two dimensional slice through a typical plasma stamp

A cursory glance at the above list readily gives one the impression that the applications of the atmospheric plasma technique are limited solely by ones imagination.  It is also clear that this short review will be able to cover only a small fraction of the material covered in this volume.  Quite likely the most innovative paper in the collection is the one on “Combinatorial Plasma-based Surface Modification …” listed as number 1 above.  This work attempts to take the process of plasma surface modification to a higher level through the use of “plasma stamps” which can be used to pattern a substrate with varying levels of plasma treatment in a single run.  A schematic diagram of a plasma stamp is shown in figure (1).  The substrate to be treated is patterned with an array of chambers using poly(dimethylsiloxane) PDMS as an insulator layer.  The resulting array is sandwiched between a porus metal mesh and an electrode.  The metal mesh in this case serves a dual purpose as a gas carrier and as an electrode.

The authors site a number of advantages of the plasma stamp configuration including:

  • Due to the small size of the plasma chambers it is easy to supply nearly unlimited volumes of gas to the active micro-plasmas which is very useful when performing film depositions as opposed to simply performing a surface modification.
  • Again due to the small cavity size the stamp can be rapidly filled using  a small amount of gas.  Thus the process is not only economical in the use of gas but the small chambers can be rapidly purged of unwanted oxygen which is a critical requirement when performing plasma nitrogenation treatments.
  • The small cavity size also allows reaction products created in the cavities that are not deposited to be swept away efficiently in the gas stream.  This is very useful in preventing fouling due to the redeposition of plasma  polymers.
  • Quite likely the most significant advantage of the plasma stamp technology is the fact that quite large arrays  of the plasma micro-cavities  can be created allowing for very efficient combinatorial studies of different  plasma treatments on a single substrate in a single run.  Thus one can easily imagine a 2 dimensional array where two different gas streams are independently introduced to the array from opposite sides of the inlet edge.  The streams will combine continuously across the entire array of cavities giving a well defined gradient of gas composition over the entire array.  Different cavities thereby receive different treatments depending on their location in the overall array.  The results can then be inspected by any of a number of surface analysis methods such as Fourier Transform Infra Red spectroscopy (FTIR) or Xray Photoelectron Spectroscopy (XPS).  Thus a large number of different surface treatments can be screened in a highly efficient manner.

Again the large scope of the volume does not allow us to comment on the other equally interesting articles. It should be clear, however, as mentioned above that the possibilities are limited only by the imagination.

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27. January 2014   9:33 am
Dr. K. L. Mittal, Dr. Robert H. Lacombe

Dr. K. L. Mittal, Dr. Robert H. Lacombe

This issue of the SURFACE SCIENCE CORNER blog inaugurates a series of essays on the above mentioned topic concerning how we recognize the ubiquitous surfaces we look at all of our waking moments.  The answer of course is through the subtle apparatus of our visual neurology which is activated by the light coming at us from all directions.  The question then becomes what information is all this light carrying to our visual cortex.  However, before we try to unravel this question we need to examine in a little more detail the nature of the light that is being reflected into our eyes from all directions.

Electromagnetic spectrum on a logarithmic scale

Electromagnetic spectrum on a logarithmic scale

The light coming at us is part of what is called the electromagnetic radiation spectrum the entire extent of which is displayed in figure (1).  The striking thing about this figure is the enormous range of the spectrum covering some 20 orders of magnitude on the logarithmic scale shown in the figure.  To print the entire figure on a linear scale would require a sheet of paper extending out to the edge of the solar system assuming 0.1 mm of sheet per 10Hz of frequency.  The second remarkable feature of this diagram for our purposes is the fact that the range of frequencies of visible radiation which is what our eyes detect is less than 2% of the range.  Though we detect a limited number of mechanical and thermal properties of surfaces through direct touch, the preponderance of our awareness of surfaces comes from reflected radiation.  Going by the figure we see that our eyes are missing some 98% of what is potentially being reflected at us.

So what are we missing in particular.  Lets consider first the infrared region from roughly 1011 to 1014 Hz.  This is radiation contributed by the incessant atomic and molecular motions of the atoms and molecules which make up all matter.  To our eyes all surfaces lying at rest are perfectly still certainly at the macroscopic level.  However, consider for example a carbon atom associated with a particular bit of organic contamination residing on some apparently undisturbed surface at room temperature.  An elementary thermodynamic calculation indicates that far from sitting still such an atom is vibrating in place at an average velocity near 300 m/sec.  This motion combined with the motion of all the other atoms and molecules on the surface gives rise to infrared radiation which is beamed in all directions and is entirely invisible to our eyes.  We can, however, detect the molecular vibrations of organic molecules on surfaces with the aid of specialized infrared spectrometers so we know they are there even though we cannot see them.

What else are we missing?  Going to higher frequencies in the range of 1016 to 1019 Hz we find ourselves in the land of the X-rays which reflect information on atomic and molecular structure on the scale of a few angstroms or roughly 10-10 cm.  If we could detect this radiation we would be able to see the atomic and molecular packing of all the species at the surface.  Things like crystal structure, grain boundaries, dislocations and assorted other types of contamination and defects lying on the surface.  In addition we would be able to detect the inherent roughness of the surface at the atomic and molecular scale.  What to the eye would appear to be a perfectly smooth surface when observed with X-rays would appear to be quite rough an rugose.  Such variable surface topography can have quite significant effects on common properties such as surface wettability.

Going to yet higher frequencies we find ourselves in the range of the gamma rays which live in the range from roughly 1019 to 1020 Hz.  The gamma rays allow us to peer into the goings on in the atomic nucleus some 1000 times smaller than the typical atom.  In particular, some nuclei are unstable and can disintegrate into smaller nuclei giving off gamma rays and other particles in the process.  Most common materials are not radioactive but some do have contamination level concentrations of radioactive species which can give off barely detectable amounts of radiation.  Now one might not expect that sub detectable levels of radiation would be of much concern to the practical product engineer manufacturing some wholly macroscopic device for industry.  However, the world of surfaces can be quite subtle and engineers in the microelectronics industry got an elementary lesson in nuclear physics from that most common of common materials lead.  It turns out that lead can harbor contamination levels of radioactive species the activity of which are quite invisible to our eyes as explained above.  Lead has a long history of being used to make electrical connections in the electronics industry going back to at least the mid 19th century.  It so happened that in the early 1980′s lead solder was being used to connect sensitive memory chips to ceramic substrates.  The memory chips were of a special kind which utilized the very high resistivity of single crystal silicon to trap a small amount of charge in a small cell which formed the basis of an elementary unit of memory.  A cell containing charge served as a boolean “1″ and an empty cell represented a boolean “0″.  All well and good but all the cells had to be connected to the remaining computer circuitry using metal lines and interconnects and of course that old work horse lead served as one of the interconnect materials.  The reader can now well guess what happened.  The radioactive contaminant species in the lead would decay from time to time giving off not only a gamma ray but also a highly charged alpha particle.  The alpha particle was the main mischief maker.  Carrying a charge of +2 it does not travel very far in ordinary matter but where it does go it leaves behind a trail of ionization which can momentarily turn a highly resistive material like single crystal silicon into a good conductor along the path of the alpha particle.  One can easily imagine a wayward alpha particle crashing into one of the silicon memory cells causing a charged cell to discharge along the ionization path left by the alpha.  A memory register of the computer has now been randomly and irreversibly changed which is not good from the point of view of programming logic.  If the affected register happened to contain an important logic instruction the result would easily lead to a serious programming error or simply machine lockup.  The field engineers came to know this type of problem as a “soft” error since the ionization trail left by the alpha particle would quickly dissipate leaving the affected memory cell quite unharmed.  Thus any attempt to locate the source of the error would be futile since no permanent hardware malfunction was involved.  Such so called “soft” errors are the worst kind from the point of view of the field engineer.  They come at random from seemingly nowhere and the culprit escapes without a trace.  How this problem was eventually solved is a story for another time but for now it simply illustrates that what we cannot see coming off a surface can indeed bite us in wholly unsuspected ways.

If we now go to the lower frequencies from 104 to 1011 Hz we come across the radio and microwave bands.  If we could detect these frequencies we would be able to see the myriad electronic and polarization currents which endlessly flow in all materials and give rise to a number of phenomena which affect us in various ways even though we cannot visually see them.

All this will be covered in future issues of the SURFACE SCIENCE CORNER blog.

The author invites any inquiries or comments.

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Category: SOLAR
11. December 2013   7:07 am
Khoren Sahagian

Khoren Sahagian

Some plasma physicists have proposed an alternative comet theory.   In their model a comet may actually be a negatively charged body created from the violent collision of large masses during planet formation.  As these charged bodies accelerate towards the sun they interact with solar winds in an extravagant display of plasma discharge.  Water or hydroxyl compounds would be the explained byproduct from the combination of the oxygen present in silicates with the protons being ejected from the sun.

Comet C/2012 S1 (ISON)

Comet C/2012 S1 (ISON) taken from TRAPPIST national telescope at ESO’s La Silla Observatory on the morning of Nov. 15, 2013. (Liège, Belgium)

Scientific observation of comets have been recorded for more than a century. In the late 20th century the scientific community reached its first consensus of the comet’s theoretical constitution.  Fred Whipple coined the hypothesis “dirty snowball” presenting the astrological object as an amalgamation of ice, rock, and star dust.  When this body nears the sun a brilliant tail emerges resulting from the sublimation of ice within the comet nucleus.

Yet some would argue that there are a few unexplained attributes of a comet to note.  First is that the coma generally always remains spherical.  This would not necessarily be expected from asymmetric jets of ice emanating from the core but might be sustained by a strong electrical field.  Second is a low constitution of water sampled in missions probing the surface and tail of a comet. One such program “Stardust mission” sent a space craft equipped with an aerogel net through the path of a comet tail.  Upon return the ground based team was surprised to find an assortment of complex high temperature crystalline formations; portions of which were anhydrous structures.  This fundamentally challenges the accepted theory as a low temperature snowball.  There are some that even liken a comet surface to objects on Earth that have become ablated by plasma discharge.  Search for SEM images and decide for yourself.

Comet; plasma or ice?

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Category: Cleaning / Glass
25. November 2013   2:31 am
Mikki Larner

Mikki Larner
Belmont, CA

Is it true that one of the first commercial uses of plasma ashing was to ablate fish to expose mercury contamination?

Sitting around the lunch table the other day, our chemist expanded on an early use of plasma for one of the first commercial applications: ashing fish to expose mercury (or other metals) to evaluate the impact of contamination from industry. While it seemed like a logical use of the technology, I couldn’t get my head around this as one of the first commercial applications….so did a bit of research and finally reached out to one of the experts in the field of vacuum technology: Donald Mattox. He confirmed that low pressure plasma ashing has been used for over 50 years for trace element analysis – an early use of replacing wet chemistry!

Don sent the following citations confirming the use:

1962: C. E. Gleit and W.D. Holland, “Use of electrically excited Oxygen for the low temperature decomposition of organic substrate” Anal Chem. Vol. 34 (11) pp 1454-1457

1977: M. Velodina, “Quantitative determination of Mercury in Organic materials by means of a low temperature, high frequency discharge plasma in oxygen” Analytical Letters 10(14) 1189-1194

And Don added one of his favorite Oxygen plasma cleaning stories (from his book “Foundations of Vacuum Coating Technology”)

When preparing to aluminize the Palomar mirror, John Strong notified the mirror polishers that he would be using a new cleaning technique using ‘a special fatty acid compound with precipitated chalk.’ When he arrived the ‘special fatty acid compound’ was Wild Root Cream Oil hair tonic (ad jingle: ‘You better get Wild Root Cream Oil, Charlie; It keeps your hair in trim; Because it’s non-alcoholic, Charlie; It’s made with soothing lanolin’). He stated, ‘In order to get glass clean you first have to get it properly dirty.’ The oil residue was ‘burned-off’ using an oxygen plasma in the vacuum deposition chamber. (From The Perfect Machine: The Building of the Palomar Telescope, Ronald Florence, pp 382-386, HarperCollins, 1994).

I’m assuming that the following US Patent from 1978 helps corroborate his story: 4088926: Plasma Cleaning Device (for cleaning organic contamination on optical surface) 

I found this quite interesting and did some additional research that I would like to share with my readers:

Plasma, atmospherically, has been used professionally by museums and NASA to remove carbon contamination or char, selectively, as a restoration technique for fine art.

Before and after image of artwork cleaned by atomic oxygen.

From http://www.nasa.gov/centers/glenn/business/AtomicOxRestoration.html

Some later work of interest was published by Texas A&M: Used RF plasma to selective remove inorganic mater from paint and prevent damage to the substrate (rock). Organic components can then be analyzed and dated.
1992: Direct Radiocarbon Dating of rock Art. Radiocarbon, V 34, No. 3, 1992, P 867-872. J. Russ, M. Hyman and M. Rowe, TAMU.

I could go on and on and on… Plasma truly offers us a tremendous tool box for modification of myriad materials!

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