S.K. Biswas
Department of Mechanical Engineering
Indian Institute of Science
Bangalore 560 012, INDIA.
For a
reversible and isothermal transformation the variation of free energy, equal to
the variation of total energy is equal to zero or negative. Using the equilibrium condition (duT=0)
condition the JKR (Johnson, Kendall and Roberts) model estimates the apparent
Hertzian load and the concomitant area of contact between two linear elastic
bodies. Corresponding to an
uniform force of attraction which exists in the contact zone, the surface
pressure is a sum of the Hertzian and the Boussinesq. There is no non contact surface force in this model,
the pressure at the contact edge remains infinite and tensile. The model yields a finite contact area
when the applied load becomes zero in the process of separation of the two
bodies. The rupture occurs on the application of a tensile load 3/2wpR
again at a finite contact area (w is the surface energy and R is the equivalent
radius of contact). The existence of Boussinesq pressure gives rise to a
discontinuity in the deformed profile just outside the edge of contact
according to this model. Further
to be consistent with the elastic nature of contact there is jump back to the
original profile at rupture.
DMT
(Derjaguin, Muller and Toporov) model gives a smooth transition back to the
original profile and registers a continuity in profile outside contact. According to this model the contact
region is under compression with a Hertzian stress distribution while outside
the contact the bodies are pulled together by interatomic forces, though the
profile remains ‘hard’ and Hertzian in this zone. Equilibrium is reached when the deformation
is such that the elastic reaction force balances the combined effect of the
surface forces and the externally applied load. The attractive force reduces from 2pRw at
low load to pRw
at large contact area. The pull
off force occurs again at point contact being equal to 2pRw.
In response to
the criticism that the DMT model does not allow for the elastic deformation of
the bodies by the surface forces Muller Yushcenko and Derjaguin in a later work
used the Lennard – Jones potential in conjunction with the applied load
to drive the deformation of bodies from the centre of contact to the non
contact periphery where the potential is asymptotically zero. Using the condition of equilibrium
specified for the DMT model (i.e force equilibrium) the deformed profile of the
bodies is derived. The pull
off force was found to be
dependent on a single parameter, m, which is a function of the body
radius, the elastic constants and the interaction potential parameter. Muller demonstrated numerically that
for small values of m i.e for stiff bodies the load-area characteristic
approaches that predicted by the DMT model while for softer bodies in contact
the solution approaches the JKR model.
This solution which bridged the JKR, DMT gap is self consistent and
showed that with increasing m the pull-off forces approaches a value of 1.5 pRw.
A third
approach due to the French workers Maugis and Barquins consider the above
as a special case of equilibrium when the energy release rate G is equal to surface energy w. Using the air gap outside the contact
as a crack they work out the stability of the system that depends on the second
derivative of the energy. The
problem in this approach is generalised as a fracture mechanics problem. In a later paper Maugis equates the stress
intensity factor due to external loading (Sneddon equations) and that due
to surface forces acting on the crack lip. This suppresses stress singularities, ensures continuity of
stresses and fixes the crack geometry.
The energy release rate is computed by the J-integral and the equilibrium
is given by G=w. The equilibrium
characteristics can now be derived for a range of axisymmetric loadings as
a function of a single parameter l which is a function of the adhesion force, surface energy,
elastic constants and the body radius. Maugis shows that when l is
varied from zero to infinity there is a continuous transition from the DMT
to the JKR approximation.
Physical
Chemistry of Interfacial Phenomena with Relevance to Tribology
Nicholas D.Spencer
Laboratory for Surface
Science and Technology
Department of Materials
Swiss Federal Institute
of Technology
ETH-Zurich, NO H64,
CH-8092 Zurich
SWITZERLAND
This lecture will
introduce the necessary concepts for an understanding of the surface chemistry
involved in tribology. The
starting point will be the idea of "clean "surfaces and the chemical
state of engineering surfaces. The
concept of the monolayer will then be introduced, as will the isotherm. The Langmuir isotherm will be
dealt with as an example.
Self-assembled monolayers will be handled as model systems for
tribological studies, using silanes, thiols, and phosphates as examples. The adsorption of larger molecules of
relevance to tribology (polyelectrolytes, proteins) will also be briefly
touched upon, as will the use of
mixed monolayers to control the hydrophobicity / hydrophilicity of the
surface. In the final part of the
lecture, the idea of boundary lubrication will be dealt with, firstly in the
context of adsorbed molecules, and secondly using examples of extreme-pressure
additives. The few examples in the
literature where the interaction of boundary lubricants with the surface under
tribological conditions has been elucidated will be examined in more detail.
Deconvolution
of Hardness from Nanoindentation Data
Musuvathi S
Bobji
Research
Fellow
University
of Oxford
Parks Road,
Oxford OX1 3PH
UNITED
KINGDOM.
The accuracy
of the hardness value estimated from a depth sensing indentation experiment
depends on many factors. When the penetration depth is very small, the
roughness of the indented surface starts influencing the measurement. The
effect of surface roughness on hardness estimation is negligible when the
penetration depths are much greater than the roughness. Thus the best way to
minimise the error is to fine polish the surface. However in many instances the
polishing would modify the surface hardness and hence is often desirable to
indent on the ‘as is’ surface.
To study the effect of the roughness, macroscopic
depth sensing experiments are carried out to simulate the single asperity and
multi-asperity contacts. The asperities were so scaled as to eliminate the
influence of depth/volume dependent strength variations. The interaction of the
strength variation with the geometry is studied numerically by simulating the
indentation on a fractal surface. Based on these studies, a model is developed
for explaining the roughness dependency of measured hardness. Given the
constants related to indenter geometry, and the surface roughness, the model
provides a means for deconvoluting the effect of roughness in arriving at real
hardness characteristics of the near surface region of a material.
Characterisation
of Surface Coatings Using the Nano-Scratch Technique; Appraisal and Examples
Dr. Nicholas X. Randall
Customer Services
Manager
CSEM Instruments
Jaquet-Droz 1
CH-2007 Neuchâtel
SWITZERLAND.
As the thickness of functional coatings continually decreases
to satisfy the structural and protective needs of modern day applications,
quantitative instrumentation has become a necessity for adequate evaluation of
material properties, particularly scratch resistance and adhesion at the
film-substrate interface. The Nano Scratch Tester (NST) is a new instrument
overcoming the limitations of both the classical stylus scratch test (normal
force range) and the scanning force microscope (SFM) technique (short sliding
distances), allowing scratch lengths of up to 10 mm. Tangential force and
penetration depth are simultaneously measured during the scratching process,
both in a multipass contact fatigue mode. For high resolution inspection of the
deformed or damaged area, a scanning force microscope (SFM) is integrated into
the system. Experimental results are presented for a range of applications
concerning ultra-thin films, as well as curved sample geometries and the use of
scratch mapping. The results indicate very good reproducibility and confirm the
application of this new instrument for the accurate characterisation of
elasticity, hardness, adhesion, friction and mechanical integrity in coated
systems where the film thickness is less than 1 µm.
Sliding and Rolling of Nanotubes and
Fullerene
Molecules
Kouji Miura
Department of Physics
Aichi University of
Education
Hirosawa 1, Igaya-cho
Kariya 448-8542
JAPAN.
The
sliding and rolling of nanotube and C60, C70 molecules on
ionic crystal and on graphite are reported. First, the bundle structure and the
sliding of single-walled carbon nanotubes (SWNTs) are reported using frictional
force microscopy [1]. The diameter of the
nanotube and the nearest distance between any two nanotubes were
estimated to be 1.4nm and 0.3nm, respectively. The frictional force required to
move SWNTs on a KCl(001) surface and its energy dissipation were estimated to
be 11nN and 0.75x10-16J, respectively. Next, it is reported that the
chirality of a multiwalled carbon nanotube (MWNT) on graphite has been determined
to be of a zigzag type using frictional force microscopy [2]. The force per
angstrom required to rotate the zigzag MWNTs in-plane on graphite has been
estimated to be approximately 4pN. Natural rolling induced by a tip contact
appears in commensurate contact with a graphite surface. The difference between
natural rolling and rolling with stick-slip lateral behavior is discussed.
In the study
on C60 and C70, the growth mode of C60 and C70
molecules on ionic crystal is first reported [3]. C60 and C70
molecules form together islands on the ionic crystal. Along different scanning
directions of
and 
of the C60(111) surface, a tip has behaved
one-dimensional stick-slip and zigzag stick-slip motions on the order of a load
of nN, respectively, although at a larger loading force it destroys the C60(111)
surface. Also, water adsorption on C60 films gave a lower frictional
force, indicating that C60 molecules rotate or translate at the
(111) surface. Thus, the C60 films are expected to exhibit various
behaviors depending on loading forces, scanning direction and relative
humidity. Next, nanotribological behavior of C60 and C70
films on graphite is reported [4]. C60 molecules on graphite begin
to grow in a monolayer form although C70 molecules begin to grow in
a bilayer form. The shear stress between a C60 monolayer and
graphite was estimated to be about 0.2GPa. At C60(111) films, there
appears a change of a tip motion from one-dimensional stick-slip to
two-dimensional zigzag stick-slip with decrease of a loading force.
Furthermore,
the behavior of C60 molecules confined by graphite substrates is
reported [5]. The lateral force of the graphite substrates confining the C60
monolayer exhibits approximately zero, originating in the rolling of aligned C60
molecules.
Reference:
[1] K. Miura et al., Appl.Phys.Lett.78(2001)832.
[2] K. Miura et al., in press to Nano Lett.1(2001).
[3] S. Okita, M. Ishikawa and K.
Miura, Surf.Sci.Lett.442(1999)L959.
[4] S. Okita and K. Miura, Nano
Lett.1(2001)101.
[5] K. Miura et
al., in preparation.
Jacob Israelachvili
Department of Chemical
Engineering and Materials Science
Associate Director of
the Materials Research Laboratory
University of California
Santa Barbara, CA 93106
U.S.A.
The following topics will be covered with
examples of their role in adhesion and tribology.
I. Particle
interactions
Basic physical forces between
molecules and small particles
A number of basic physical interaction
forces occur between all molecules and particles: these are the attractive van
der Waals force and, depending on the nature of the materials, electrostatic
(Coulombic, ionic and charge exchange) forces and, between metals, short-range
metallic bonding forces. Non-physical forces, such as chemical or covalent
bonds, will not be considered in these talks.
Interactions in a liquid medium
versus in air
The presence of a liquid or other
condensed phase between two molecules or particles, i.e., for interactions in a
medium, the forces can be very different, both quantitatively and
qualitatively, e.g., repulsive rather than attractive. In addition, new types
of forces that do not occur in air or vacuum can arise, such as repulsive
entropic or steric forces, oscillatory solvation or structural forces, and
attractive hydrophobic forces.
Short-range (adhesion) and
long-range (colloidal) forces
The effects of a force at short-range can
be very different from its effect at long-range. Short-range forces determine
adhesion, the strengths of materials, and friction forces. Long-range forces
determine the stability of colloidal particle dispersions and their rheological
behavior. In the case of extended surfaces (see Lecture II) they determine the
equilibrium thickness of thin liquid films and lubrication forces.
II.
Surface interactions
Differences between small particles
and extended surfaces
The strength of an interaction usually
scales with the radii of the interacting particles and, between two flat
surfaces, with their ‘contact’ area. However, there are other
important differences that depend on the geometry of the interacting particles
and, in the case of surfaces, on their smoothness or ‘topography’.
The bulk elastic properties of adhering
or sliding surfaces can also have a dramatic effect on their adhesion and
friction forces.
Surface thermodynamics
For extended surfaces, which necessarily
involve the simultaneous interaction of many molecules, certain useful
thermodynamic principles and equations can be applied. These involve parameters
such as the surface tension, surface energy, interfacial energy, work of
adhesion, work of cohesion, and contact angles, all of which provide useful
information on the adhesion and friction forces of ‘ideal’
molecularly smooth surfaces.
Rough versus molecularly smooth
surfaces
Real, engineering surfaces are
usually rough rather than molecularly smooth. The roughness can be at the
nanometer scale or at the micron scale. Current thinking on the effects of
surfaces roughness on adhesion and friction will be described.
Surface Enhanced Raman Scattering
of Monolayers
Ajay K. Sood
Department of Physics
Indian Institute of Science
Bangalore-560012
INDIA
Raman spectroscopy has proved to be
a very powerful probe to characterize the nature and chemical environment of
molecules in different phases. About twenty five years ago, it was discovered
that Raman scattering associated with the vibrational modes of molecules
increases many fold (~ 106) when these are adsorbed on the rough
metal surfaces. This phenomena, termed as Surface Enhanced Raman Scattering
(SERS) has a tremendous potential in studying monolayers adsorbed on the metal
surfaces through the vibrational fingerprints under different conditions.
This
talk will present an introduction to SERS covering basic experimental facts and
the essential features of the mechanisms proposed to account for the
observations. The main contribution to the SERS enhancement
arises from the strongly enhanced optical fields near the rough metal surface.
A spherical particle whose radius is much smaller than the wavelength of light,
experiences an electromagnetic field at the surface given by Einduced
= {[e1(w) - e2]/[e1(w) + 2e2]}Eexternal
, where e1(w) is
the complex, frequency dependent dielectric function of the metal and e2
is the relative permittivity of the ambient phase. It is clear that Einduced is very large when e1(w) =
-2e2. An excitation of the
surface plasmon greatly enhances the local field experienced by the molecule
adsorbed on the surface of the metal particle. Another contribution called
chemical enhancement comes from the interaction/charge transfer between the
molecule and the metal. It can be explained as a resonance Raman mechanism in
which either the electronic states of the adsorbates get shifted or broadened
due to interaction with the metal surface or new electronic states arise due to
chemisorption serving as intermediate states for resonance Raman scattering.
The case studies to be presented include self-assembled monolayers formed from
alkanethiols and single wall carbon nanotubes. It will be shown that polarized
SERS can give information on the orientation of the molecules adsorbed on the
surface.
Rationale for Additive Formulations
D.K. Tuli , R. Sarin
and A. K. Bhatnagar
Indian Oil
Corporation Ltd.,
Research and development Centre
Faridabad -121 007
INDIA.
Chemical
additives are incorporated in lubricating base stocks to enhance or to create
desired properties vital for meeting the lubrication demands of the mating
surfaces. A large number of chemical functionalities are in use in various
lubricant formulations which improve the load carrying properties, reduce
friction and wear, lower corrosion etc,. Most of these additives are
surface-active molecules and affect the performance of other additives. The
performance of additive mixture cannot be predicted in a simple fashion, for
example, such as, by rule of mixture approach. Basic understanding of the absorption / adsorption processes
of each individual additive on the metallic surfaces provides the much needed
inputs for the selection of base oils , additives and for formulating a
product. Therefore, it is necessary to understand how additives work and to be
able to predict how other molecules in the similar conditions would function.
However, despite several empirical studies, actual chemical mechanism by which
most of the additives function remains poorly understood. Thus, there is a need
to correlate molecular level information to macroscopic tribological behavior.
The
current lubricant formulation technology derives its strength from the bulk
tribology studies followed by performance in the field. Though this formulation
methodology had been delivering but the additive dose optimization and right
additive selection sometimes becomes causality because of lack of understanding
of the nanotribology of the additives. A systematic approach for the lubricant
formulation technology should address a) Molecular design- selection of
relevant additive structure ; b) Composition design - formulation of the
required lubricant composition and d) Performance design - prediction of
lubricant functional performance.
Study
of functional properties of pure additive components could lead to much needed
structure - performance relationships. Generation of large number of additive
molecules by adopting the combinatorial Chemistry methods followed by tribo
evaluation could create data banks for optimization of lubricant quality,
savings on testing times while meeting the specific lubrication needs.
Chemistry
of some of the commonly used additives in lubricant formulations shall be
discussed and gap areas shall be focused.
Measurement
of Forces between Individual Emulsion Droplets
John Philip,
T.Jayakumar, P.Kalyanasundaram and Baldev Raj
Metallurgy &
Materials Group
Indira Gandhi Center
for Atomic Research,
Kalpakkam-603102,
Tamilnadu
INDIA.
O.Mondain Monval and
J.Bibette
CRPP-CNRS, Avenue Albert
Schweitzer, 33660, Pessac,
FRANCE.
Applications
of colloidal science are in constant progress in many aspects of life. Colloids are used in a large number of
processes such as extracting oil from geological deposits, aerosols for
dispensing domestic products like shaving creams and deodorants, sprays
containing insecticides,
emulsions, gels, food processing, packaging industry etc. Colloids generally
require repulsive surface forces to become metastable. The net force acting
between the colloidal particles determines the stability of the colloidal
system. A net attractive force leads to the aggregation or clustering of
particles or liquid particle coalescence may occur and the system becomes unstable. Measurement of forces acting
between solid surfaces has been a topic of intense research over the last two
decades. Recently, a new
technique, called Magnetic field induced Chaining Technique (MCT) has been
introduced which allows the measurement of forces between two individual
emulsion droplets. In contrast to other older techniques such as surface force
apparatus, atomic force microscopy, total internal microscopy etc, this method leads to the direct
"in- situ” measurement at the colloidal scale. We discuss the
details of the newly developed force measurement technique, which facilitates
the measurement of the forces between individual colloidal particles. The
results on the forces between individual colloidal liquid droplets in the
presence of different types of surfactants and macromolecules are presented.
Chemical
Analysis of Interfaces: Spectroscopy
Nicholas D.Spencer
Laboratory for Surface
Science and Technology
Department of Materials
Swiss Federal Institute
of Technology
ETH-Zurich, NO H64,
CH-8092 Zurich
SWITZERLAND
This lecture will focus
on three commonly used surface analytical methods: Auger electron spectroscopy
(AES),secondary ion mass spectroscopy (SIMS),and x-ray photoelectron
Spectroscopy (XPS or ESCA). AES is
a surface-analytical method that yields the elemental composition of the top
few nanometers of a surface.
Under certain circumstances, it can also be used to obtain some
chemical-state information. Mostly
applicable to conducting substrates,
it produces highly quantitative information with a spatial resolution of
tens of nanometers. SIMS is
a highly surface-sensitive method, yielding chemical structural information on
the outer molecular layer of a sample.
Semi-quantitative at best, it is particularly useful when applied in
tandem with other techniques. SIMS can be used to obtain
chemical images with a spatial resolution of around 1 µm. XPS is a versatile method for the
chemical analysis of surfaces.
Like AES, it has a surface sensitivity of a few nanometers, but unlike
AES, it can yield invaluable chemical-state information, and is applicable to
the analysis of both conductors and insulators. It can also be used for imaging, but generally with a resolution of some tens of
micrometers. All three
methods will be dealt with in the context of tribology and the study of
lubricant-surface interactions.
Particular emphasis will be placed on applications of these techniques
that involve imaging.
Friction
Anisotropy and Lubricant Layering Effects at Single Crystalline Interfaces
Andrew J. Gellman
Lord Professor of
Chemical Engineering
and Professor of
Chemistry
Carnegie Mellon
University
Pittsburgh, PA 15213
U.S.A.
Atomic
scale influences on the macroscopic tribological properties of surfaces are
being explored through studies of friction between single crystalline and
quasicrystalline surfaces prepared and characterized under ultra-high vacuum
conditions. Our experiment allows
us to make friction measurements between pairs of well prepared and very highly
characterized surfaces to be brought into contact under an applied load and
then sheared at constant velocity.
Before the friction measurements the surfaces can be cleaned, oriented,
and analyzed suing Low Energy Electron Diffraction (LEED) and Auger Electron
Spectroscopy (AES). This allows us
to create highly idealized interfaces.
One simple set
of measurements has explored the frictional properties of single grain
quasicrystals. Quasicrystals are
materials have very complex, aperiodic structures with five-fold rotational
symmetry axes and claims have been made that they exhibit anomalously low
frictional properties. Under
vacuum conditions Al70Pd21Mn9 quasicrystal
surfaces have been cleaned prior to making friction measurements. While these yield coefficients of
friction for perfectly clean surfaces that are significantly lower than those
observed with clean metal surfaces they are higher than those measured with
quasicrystals in air. Controlled
oxidation of the Al70Pd21Mn9 quasicrystal
surfaces has been shown to lower friction coefficients and be in part the
source of the low friction measured in air. One interesting comparison has been made between the
frictional properties of the quasicrystal and an approximant phase that has a
similar composition but a periodic bulk lattice. This shows that the friction between the quasicrystals is
truly lower that those of similar periodic compounds.
Additional and
more complex measurements have explored the frictional anisotropy between
single crystal Ni(100) surfaces as they are rotated with respect to one
another. These experiments probe
the relative importance of surface lattice versus bulk lattice orientation in
determining friction between metal crystals. The results show that there is anisotropy in the friction
but that this cannot be associated with the relative orientations of the
surfaces lattices. Rather it is a
properties of the relative orientations of the bulk crystal lattices.
Finally,
we have explored the effects of adsorbed films on the friction between metal
surfaces. The Ni(100) surfaces
have been modified by the adsorption of ½ monolayer of sulfur atoms in a
c(2x2) array. The adsorption of
ethanol on these surfaces has been shown to result in layers with heats of
adsorption that vary discontinuously with coverage. The interesting observation is that the friction at the
Ni(100)/Ni(100) interface lubricated with these ethanol layers reveals
discontinuities at the same coverages as the discontinuities in the heats of
adsorption. These show evidence
for the layering of molecular films in sliding contacts and for the effects of
layering on friction.
Molecular
Simulations of Fluid Structure in Smooth and Rough Slit Nanopores
K G. Ayappa and
Chandana Ghatak
Department of
Chemical Engineering
Indian Institute of Science
Bangalore 560012
INDIA.
The
structure of spherical Lennard-Jones (LJ) fluids confined in smooth and rough
nanopores is investigated using grand canonical Monte Carlo simulations for
three different bulk liquid conditions. One of these bulk state points lies
close to the liquid-solid line of the LJ phase diagram. Simulations in the smooth pore
corresponding to bulk state point lying close to the liquid-solid line, reveals
that the fluid not only freezes into the pore at pore heights that correspond
to a maxima in the solvation force, but also undergoes a solid-solid transition
from a square to a triangular lattice for small changes in pore height. This
transition occurs at pore heights that accommodate two and three fluid layers.
Classification of solid structures based on the 3D counterparts, indicate that
the transition occurs between solids that correspond to a body centered
tetragonal (bct) structure to the hexagonal close packed (hcp) or face centered
cubic (fcc) structure.
Rough
pores were constructed by introducing sinusoidal undulations on the top wall of
a smooth pore. Roughness was
varied by changing the amplitude and wavelength of the undulation. Two kinds of rough pores were
investigated. In RPA a 10-4 potential is used to model the fluid-wall
interaction, where the interaction is only a function of the normal fluid-wall
distance. In RPB the walls are made up of discrete atoms which interact with
the fluid via a 12-6 LJ potential.
We observed an overall decrease in the amplitude of oscillations in the
solvation force curve for the fluid confined in the rough pores and a phase
shift in the solvation force curve for certain values of the amplitude. In both RPA and RPB at the smaller pore widths and low values of amplitude
the presence of the undulations in the top wall induced a greater degree of
layering in the fluid on the bottom wall. In the case of RPA fluid particles
tend to align in a direction
parallel to the undulations in the top wall at a particular value of amplitude
and wavelength which could not be predicted a priori. This preferred alignment
of fluid particles did not occur in the more realistic pore model RPB.
Electrochemical
Characterization of Self-Assembled Thin Films
S. Sampath
Department of Inorganic
and Physical Chemistry
Indian Institute of
Science
Bangalore 560 012
INDIA.
Many
organic molecules are known to form well-ordered, densely packed and
crystalline like assemblies on various metal substrates like gold, silver, iron
etc. The physical and chemical properties of these assemblies are generally
followed by IR, Raman, XPS, Ellipsometry and electrochemical techniques. Among
these, electrochemical techniques yield information on the modified surfaces
that are in contact with a liquid (electrolyte).
In
the present study, we have characterized a variety of mono- and multilayers on gold, silver and platinum substrates
using electrochemical techniques. The amphiphiles used are thiols, thiols with
an electroactive moiety at the end, phthalocyanines and silanes. The physical
properties such as coverage, orientation, thickness of the film, pKa of the
terminal ionisable groups, dielectric constant have been deduced using cyclic
voltammetry and impedance techniques. We have also followed the phase
transitions occurring in self-assembled monolayers, as a function of
temperature. Some of these results will be presented and discussed.
Stability, Dynamics and Morphology
of Thin Liquid Films
Ashutosh Sharma
Department of
Chemical Engineering,
Indian Institute of
Technology,
Kanpur 208016,
INDIA
Thin (10 -100 nm) liquid films
are of increasing scientific and technological importance, e.g., in areas as
diverse as coatings, paints, adhesives, flotation, wetting, tribology,
microelectronic and optical devices, nano-technology and ophthalmic products
for “dry eyes”. This talk will provide a brief overview of recent
advances in our understanding of the stability, dynamics and morphology of thin
liquid films, as well as of the open questions in the area. Relatively thinner
films on partially wettable homogeneous substrates dewet by the formation of
droplets, whereas films thicker than a critical thickness breakup by the
formation of isolated circular holes.1 This finding has challenged a
long standing belief that the holes are always indicative of a
“nucleation” mechanism. Even the thin films of the fluids that
display perfect macro-scale wetting can
dewet to produce equilibrium morphologies resembling either a
“pancake” or a “Swiss-cheese” like membrane structure.
The nature of morphological patterns during dewetting contains signatures of
the underlying long and short-range intermolecular interactions. Based on this
concept, an enigmatic long range force, which mimics the decay of van der Waals
force but is about three orders of magnitude stronger, has been detected in the
polymer films sandwiched between an aqueous media and a silicon wafer.2
That dewetting in our PDMS system indeed occurs by the spinodal mechanism was
confirmed by a direct test of both the length and time scales.2,3 A new mechanism of surface instability has been
identified on chemically heterogeneous surfaces.4 Even small
wettability gradients engender the breakup of very thick (~ 100
nm) and very viscous polymer films within a few minutes by the formation of
complex, locally ordered patterns. Other fascinating aspects of thin films
concern the instability of moving contact lines. An evaporating water drop
develops holes at its periphery, whereas retraction of a PDMS film engenders
long finger like dynamic ridges that continually form and fragment into
droplets.
Reference:
1 A. Sharma and R.
Khanna, Phys. Rev. Lett. 81,
3463 (1998); J. Chem. Phys. 110,
4929 (1999).
2 G. Reiter, A.
Sharma, et al., Langmuir 15, 2551 (1999); Europhys. Lett. 46, 512 (1999).
3 G. Reiter, R. Khanna
and A. Sharma, Phys. Rev. Lett. 85,
1432 (2000).
4 Konnur
R., Kargupta K. and Sharma A., Phys. Rev. Lett. 84, 931
(2000); Langmuir 16, 10243
(2000).
Physics
of SPM and Outstanding Issues
John B. Pethica
Professor of Materials
Science
University of
Oxford, Dept. of Materials
Parks Rd., Oxford OX1
3PH
UNITED KINGDOM.
The talk will first
address the present understanding of the interactions between tip and surface
which underlie the operation of scanned probe microscopes, with particular
reference to STM and AFM. These interactions, whose short range is the key to
imaging resolution, can in principle be studied directly by I-V or
Force-distance spectroscopy, and the combination of spectroscopy with imaging
is a particularly powerful aspect of SPM.
In STM spatial variation
of density of states can be found from I-V and related spectra, and examples
will be cited from metals, semiconductors and oxides. Uncertainties remain in
the role of tip electronic structure, the 3-D nature of the tunnel current, and
the effect of tip deformations on imaging. In the case of non-contact AFM, the interactions are all
summed to give the measured force.
Thus tip structure, and hence relative proportion of long (Van der Waals etc.)
and short range (covalent, atom electrostatic) affects image contrast. Some
examples of mapping force gradients at single atoms will be shown. New
measurements of the onset at small separations of dissipative atom
displacements will be described. This will be related to controlled atom
manipulation techniques. Finally,
contact mode AFM will be discussed. Both experiment and MD simulation suggest
that standard contact mechanics techniques are applicable to such tribological
junctions once the scale exceeds that of a few atoms or molecules. The talk
will finish with a few results from the intermediate, transitional length
scale, and some problems with their interpretation.
AFM
for Nanotribology Measurements
Suzanne P
Jarvis
Nanotechnology
Research Institute,
1-1-4 Hiashi, Tsukuba
JAPAN.
Atomic force microscopy (AFM) evolved from the
observation of the effects of mechanical contact during scanning tunnelling
microscopy imaging. Thus, even
from its first inception, tribology and atomic force microscopy have been inextricably
linked. Whilst not all
tribological phenomena have been intentional or welcomed in AFM measurements
there has also been a concerted effort to apply AFM to the field of
nanotribology. Due to its
highly localised measurement ability, the microscope can be applied to a
variety of tribological processes which are too laterally specific to be
investigated by Surface Forces Apparatus or nano-indentation devices.
At the core of this lies the microscope's
ability to form single asperity contacts a few nanometres across due to the very sharp probe tip, shown in
the figure below. However, in order to be usefully applied, AFM
measurements must be quantified and the identity of the tip and sample
material must be known. These features are not trivial due to
the design and environment of most microscopes and the scale of the measurements
where even minute traces of contamination can have noticeable impact on
quantitative measurements.
Figure (a) SEM micrograph of a typical
microfabricated silicon cantilever showing the basic measurement technique
employed in AFM. (b) SEM micrograph of the sharp tip of a silicon sensor with
ultrasharp carbon nanotube tip attached.
In most standard instruments forces are not
applied directly. Instead
displacements are usually applied to the sample and forces are often calculated
from the multiplication of roughly calibrated displacements and the arbitrarily
known spring constant of the force sensor. The use of such measurements has lead to widespread
scepticism of the application of AFM to tribological problems. However, various calibration methods
with both a theoretical and experimental basis can be employed and new
experimental techniques and modifications have been implemented which greatly
improve the credibility of the instrument as a measurement device for
tribological processes.