Interfacial Force Microscope Research Articles

Temperature dependent relaxation of a “solid–liquid”

AbstractWe have used the Interfacial Force Microscope to perform temperature dependent indentation measurements on a model viscoelastic material, Silly Putty. By transforming time dependent stress relaxations into frequency dependent modulus, we can identify the temperature dependence of the elastic and viscous response of an experimentally challenging material. This technique promises to be useful in determining the mechanical properties of composite materials with microscopic spatial resolution. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1285–1290, 2009

An electrostatically actuated cantilever device capable of accurately calibrating the cantilever on-chip for AFM-like applications

We present the principle of an electrostatically actuated cantilever device that can calibrate the cantilever stiffness on-chip with uncertainties in the range of ±5%. The calibration procedure is quick, simple and non-destructive. The device can be fabricated using routine micromachining techniques and be easily made compatible with commercially available atomic force microscopes (AFMs). The electrostatic actuation makes the device quite versatile. In addition to the usual AFM-like applications, the device can also be used for mass-sensing applications as well as an effective interfacial force microscope.

Nanomechanics of biocompatible TiO 2 nanotubes by Interfacial Force Microscopy (IFM)

Titanium dioxide (TiO 2) coatings exhibit desirable properties as biocompatible coatings. In this paper we report on mechanical properties and deformation behavior of (TiO 2) nanotubes grown on pure titanium substrates through anodic oxidation. Characterization of the as-processed coatings was conducted using scanning electron microscopy (SEM). Nanoindentation, using Interfacial Force Microscopy (IFM), was employed to probe the Young’s modulus of the nanotubes. Using the IFM technique, the modulus of the nanotube coating may be measured with minimal contribution from the underlying Ti substrate. The modulus of the (TiO 2) nanotube coating was estimated at 4–8 GPa. This technique was also used to study the inelastic deformation behavior of the nanotubes. (TiO 2) nanotubes were found to inelastically deform by “tube crushing” in the immediate vicinity of indenter tip, increasing the local density. This increase in local density caused an increase in the Young’s modulus from roughly 4 GPa to 30 GPa in the first 30 nm of indentation. Densification and the resulting increase in elastic modulus are related to the total work of inelastic deformation, irrespective of the loading history.

Interfacial Characterization of β-Lactoglobulin Networks: Displacement by Bile Salts

The competitive displacement of a model protein (beta-lactoglobulin) by bile salts from air-water and oil-water interfaces is investigated in vitro under model duodenal digestion conditions. The aim is to understand this process so that interfaces can be designed to control lipid digestion thus improving the nutritional impact of foods. Duodenal digestion has been simulated using a simplified biological system and the protein displacement process monitored by interfacial measurements and atomic force microscopy (AFM). First, the properties of beta-lactoglobulin adsorbed layers at the air-water and the olive oil-water interfaces were analyzed by interfacial tension techniques under physiological conditions (pH 7, 0.15 M NaCl, 10 mM CaCl2, 37 degrees C). The protein film had a lower dilatational modulus (hence formed a weaker network) at the olive oil-water interface compared to the air-water interface. Addition of bile salt (BS) severely decreased the dilatational modulus of the adsorbed beta-lactoglobulin film at both the air-water and olive oil-water interfaces. The data suggest that the bile salts penetrate into, weaken, and break up the interfacial beta-lactoglobulin networks. AFM images of the displacement of spread beta-lactoglobulin at the air-water and the olive oil-water interfaces suggest that displacement occurs via an orogenic mechanism and that the bile salts can almost completely displace the intact protein network under duodenal conditions. Although the bile salts are ionic, the ionic strength is sufficiently high to screen the charge allowing surfactant domain nucleation and growth to occur resulting in displacement. The morphology of the protein networks during displacement is different from those found when conventional surfactants were used, suggesting that the molecular structure of the surfactant is important for the displacement process. The studies also suggest that the nature of the oil phase is important in controlling protein unfolding and interaction at the interface. This in turn affects the strength of the protein network and the ability to resist displacement by surfactants.

Friction, Wear, and Aging of an Alkoxy-monolayer Boundary Lubricant on Silicon

We study the friction, wear, and aging of a model boundary lubricant, an alkoxy monolayer covalently bonded to a Si(111) surface, using an interfacial force microscope with a spherical diamond probe. The robust alkoxy bond creates a film that effectively lubricates and prevents wear of Si at stresses comparable to those found in microelectromechanical systems devices. Sliding on the monolayer over 50 nm produced friction approximately three times greater than that of sliding over molecular length scales (∼2 nm); this is attributed to deformation dynamics of the experiment. By repeated scanning over the same location, we observed wear on a level that reduces the friction by thinning and/or reordering the monolayer film.

Cantilever based optical interfacial force microscope

We developed a cantilever based optical interfacial force microscopy (COIFM) that employs a microactuated silicon cantilever and optical detection method to establish the measurement of the single molecular interactions using the force feedback technique. Through the direct measurement of the COIFM force-distance curves, we have demonstrated that the COIFM is capable of unveiling structural and mechanical information on interfacial water at the single molecular level over all distances between two hydrophilic surfaces.

Density Dependent Friction of Lipid Monolayers

We measure frictional properties of liquid-expanded and liquid-condensed phases of lipid Langmuir-Blodgett monolayers by interfacial force microscopy. We find that over a reasonably broad surface-density range, the friction shear strength of the lipid monolayer film is proportional to the surface area (42-74 A2/molecule) occupied by each molecule. The increase in frictional force (i.e., friction shear strength with molecular area can be attributed to the increased conformational freedom and the resulting increase in the number of available modes for energy dissipation.

Nanospot welding and contact evolution during cycling of a model microswitch

The useful lifetime of microelectromechanical system switches is shortened during repetitive contact when the continual making and breaking of an electrical circuit accelerates damage done to the metallic contact points in the switch. In this study the interfacial force microscope is used as a model switch, and we explore the fundamental processes involved in switch failure. We find that repeated indentation (cyclic contact) causes protective coatings (in the form of self-assembled monolayers) to fail allowing metal-metal intimacy and formation of a malleable “nanospot weld.” The weld is stretched during separation of the contacting surfaces, leading to the development of nanoasperities. With the help of atomistic simulations, which provide insight into material transfer and consequential roughening of the surfaces, we show that asperity length grows with continued repetition, drastically changing the resistance of the contact over the lifetime of the switch. Controlling the amount of current passed through the contact influences the extent of weld stretching and arcing observed during contact separation.

Does Exceptional Viscous Drag Impede Flow Through a Nano-Sieve's Pores?

ABSTRACTTo address the concern that a material functionalized to reject dissolved ions may give rise to a highly viscous aqueous interphase within nanometers of its surface, we conducted and interpreted Interfacial Force Microscope measurements of the dissipative forces that resist motion of a tip parallel to a sample, in water. The results are consistent with earlier measurements where the tip approached the sample. They confirm that near hydrophilically functionalized surfaces, interphase viscosities more than a million times that of bulk water can be expected.

A local‐probe analysis of the rheology of a "solid liquid"

AbstractResults are shown demonstrating the application of the interfacial force microscope to a study of the micro‐scale mechanical properties of an extreme example of a viscoelastic material, one that is often referred to as a “solid liquid.” Experiments involve relaxation measurements taken over a range of deformations, to establish linearity, and scaled according to the optically determined contact radius. In addition, the data is Fourier analyzed to obtain the frequency response of both the real and imaginary components of the shear modulus. The results from such an analysis of a single 3 s measurement are shown to be in remarkable agreement with published results from a series of single‐frequency measurements using a classical rheometer. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2993–2999, 2005

Self-Assembled Silane Monolayers: Fabrication with Nanoscale Uniformity

Illustrating direct connections between surface chemical events and mechanical and topological characteristics of self-assembled monolayers derived from octadecyltrichlorosilane (OTS) adsorption on Si(100), layers prepared in the presence and absence of moisture have been characterized. Uniform and robust self-assembled monolayers are demonstrated provided the Si(100) surface is fully hydroxylated by treatment in piranha solution and the dried surface is exposed to OTS under strict anhydrous conditions. With nanoscale resolution, the uniform mechanical properties are confirmed by interfacial force microscopy while the uniform topological properties are evident in atomic force microscopy images. The monolayer character of the OTS coverage is confirmed by X-ray photoelectron spectroscopy, ellipsometry, and patterning experiments. Analogous surfaces, prepared in the presence of moisture, exhibit nonuniform topological and mechanical properties.

A Hybrid Continuum-Molecular Analysis of Interfacial Force Microscope Experiments on a Self-Assembled Monolayer

Nanoindentation experiments were performed on a defect-free, molecular self-assembled monolayer of octadecyltrichlorosilane (OTS) on silicon using an interfacial force microscope (IFM). The IFM provided repeatable and elastic force profiles corresponding to the adhesive and compressive response of these 2.5nm thick monolayers. As a first step in the analysis of the force profiles, the OTS was assumed to be linearly elastic and isotropic, and adhesive interactions were accounted for via a cohesive zone model. However, the assumption of linearity gave rise to force profiles that did not match the measurements. As a result, the mechanical behavior of the OTS was extracted from molecular-dynamics simulations and represented as a hypoelastic material, which, when used in finite element analyses of the IFM experiments, was able to fully reproduce the force profiles. This suggests that the continuum representation of the mechanical and adhesive behavior of self-assembled monolayers may be directly obtained from molecular analyses.

Nanoindentation of polymeric thin films with an interfacial force microscope

The mechanical properties of interphase regions at bi-material interfaces can be quite different from the surrounding bulk materials. For composite materials, this interphase region is usually thin but plays an important role in their overall mechanical properties. Nanoindentation has become a commonly used experimental technique for measuring the mechanical properties of materials, especially when one of the dimensions is small. However, the extraction of reduced elastic modulus from the nanoindentation of thin films on substrates can pose challenges due to the influence of the substrate. In this study, the nanoindentation of thin films on substrates has been examined with a view to extracting the reduced modulus of thin polymer films. Thin films of (3-aminopropyl)triethoxysilane (C 9H 23NO 3Si, γ-APS) were deposited on silicon. An interfacial force microscope (IFM) was used to indent the γ-APS films. The effect of the substrate was studied by considering two very different thicknesses ( 4 μm and 46 nm ). The nanoindentation data were analyzed via contact mechanics theories and a finite element analysis that incorporated surface interactions. The analyses showed that nanoindentation experiments can provide reliable values of film modulus when the film is very different from the substrate. It was found that the commonly used rule of thumb that the indentation depth should be less than 10% of the thickness did not eliminate substrate effects for a wide range of material combinations. Instead, it is proposed that the contact radius should be less than 10% of the thickness so that contact mechanics theories for monolithic materials can be used without considering the presence of the substrate. The modulus of γ-APS polymer films and the surface energy between the tungsten tip of the IFM and γ-APS films were extracted and were related to their cure. A completely cured 46 nm thick γ-APS film had a reduced modulus of 3.5±0.5 GPa . This value falls in the usual range for polymers due to the amorphous nature of the γ-APS films.

Effect of High-Viscosity Interphases on Drainage between Hydrophilic Surfaces

Drainage of water from the region between an advancing probe tip and a flat sample is reconsidered under the assumption that the tip and sample surfaces are both coated by a thin water "interphase" (of width approximately a few nanometers) whose viscosity is much higher than that of the bulk liquid. A formula derived by solving the Navier-Stokes equations allows one to extract an interphase viscosity of approximately 59 kPa x s (or approximately 6.6 x 10(7) times the viscosity of bulk water at 25 degrees C) from interfacial force microscope measurements with both tip and sample functionalized hydrophilic by OH-terminated tri(ethylene glycol) undecylthiol, self-assambled monolayers.

Direct Observation of Photo Switching in Tethered Spiropyrans Using the Interfacial Force Microscope

The interfacial force microscope has been used to establish the changes in the surface chemistry that accompany photoactivated opening and closing of rings in tethered spiropyran monolayers. Contact potential measurements show that ring opening via ultraviolet light creates a more polar surface, while exposure to visible light closes the rings and resets the surface. Normal force measurements show that open ring structures are protonated in electrolyte solutions, amplifying the switching of electrical double-layer forces.

Viscous “Interphase” Water Adjacent to Oligo(ethylene glycol)-Terminated Monolayers

We report measurements of the interaction force between functionalized probe tips and oligo(ethylene glycol) (OEG)-terminated monolayers. Force profiles obtained with the interfacial force microscope indicate the presence of an interphase water layer adjacent to the OEG-terminated monolayer that generates a repulsive hydration force. This interphase layer extends up to 5 nm from the surface and has a viscosity that is 6 orders of magnitude greater than that of bulk water. The possible role this interphase layer plays in the exceptional protein resistance of such systems is discussed.

A single asperity study of Au/Au electrical contacts

Interfacial force microscopy (IFM) is used to measure the electrical contact properties of electroplated gold thin films of the type used in microelectromechanical system relays. Force and current levels consistent with those present in metal-metal contact switches are examined in an atmospheric-pressure, dry-nitrogen ambient at room temperature, and the nature of a nonmetallic contamination layer which limits contact resistance and lifetime is explicitly examined mechanically, electrically and chemically. The electrical and mechanical properties of the contamination layer on the gold substrate are observed by IFM both before and after being exposed to ozone for an extended period of time. The contamination film is characterized by x-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry, and found to consist mostly of hydrocarbons; the film remains relatively stable in both composition and thickness following ozonation. However, some subtle chemical changes in the contamination layer induced by the ozonation process are found to profoundly affect the electrical properties of the gold-gold contact, reducing the resistance by more than 3 orders of magnitude and considerably reducing variability in the contact resistance between contact events. These results clearly demonstrate the critical role both positive and negative of the latent contamination present on the contact surfaces.

Chemical and Mechanical Properties of ZDDP Antiwear Films on Steel and Thermal Spray Coatings Studied by XANES Spectroscopy and Nanoindentation Techniques

X-ray Absorption Near Edge Structure (XANES) has been used to characterize the chemistry of tribochemical wear pads generated from a paraffinic base oil with a zinc-dialkyl-dithiophosphate additive on steel surfaces and high-velocity oxygen fuel (HVOF) thermal spray coatings. The phosphorus K- and L- edge XANES spectra show that the tribofilms formed on steel and the HVOF coatings have the same chemical nature. Also, mechanical properties of these tribofilms were examined by nanoindentation techniques using both Hysitron and Interfacial Force Microscopy (IFM) instruments. The elastic moduli extracted from indentation force-displacement (f-d) curves have demonstrated that tribofilms on steel and HVOF coatings have similar surface topography and mechanical properties.

Tribological Properties of Alkoxyl Monolayers on Oxide Terminated Silicon

The tribological properties of alkoxyl monolayers on oxide terminated silicon surfaces have been investigated using interfacial force microscopy. For a C18 alkoxyl monolayer, both adhesion and frictional properties are similar to those of a self-assembled monolayer of octadecanethiol on gold. Friction is shown to increase as the alkyl chain length of the molecules decreases. Analysis using contact mechanics models has been carried out to estimate reduced modulus, adhesion energy, and friction shear strength. These interfacial mechanical properties are correlated to molecular structures at the interface.

Adhesion, friction, and mechanical properties of functionalized alkanethiol self-assembled monolayers.

We have used interfacial force microscopy to study the adhesion, friction, and mechanical properties of molecular monolayers self-assembled on Au surfaces. This quantitative and stable scanning-probe technique permits detailed studies of these factors. By systematic variation of the chemical nature of the end groups on the monolayers and utilization of standard and intuitive contact-mechanics models, quantitative results are presented of inter- and intrafilm bonding strength as well as the relationship between mechanical behavior and the lateral friction force.