Nitya Nand Gosvami

Self-assembly in the electrical double layer of ionic liquids.

We have studied the structure of two ionic liquids confined between negatively charged mica sheets. Both liquids exhibit interfacial layering, however the repeat distance is dramatically different for the two liquids. Our results suggest a transition from alternating cation-anion monolayers to tail-to-tail cation bilayers when the length of the cation hydrocarbon chain is increased.

Mechanisms of antiwear tribofilm growth revealed in situ by single-asperity sliding contacts

Additive explanation for anti-wear Additives in oil are vital for protecting engines from wear by forming films at sliding interfaces. Zinc dialkydithiophosphate (ZDDP) has been used for decades to reduce engine wear. Now there is a strong incentive for finding a replacement for ZDDP: Its breakdown products shorten catalytic converter lifetime. Gosvami et al. examined exactly how ZDDP produces an anti-wear film under high stress or elevated temperature (see the Perspective by Schwarz). Understanding these mechanisms will help in the development of higher-performance and more effective additives. Science, this issue p. 102; see also p. 40 Antiwear properties of an oil additive stem from tribofilms that develop under elevated stresses and temperatures. [Also see Perspective by Schwarz] Zinc dialkyldithiophosphates (ZDDPs) form antiwear tribofilms at sliding interfaces and are widely used as additives in automotive lubricants. The mechanisms governing the tribofilm growth are not well understood, which limits the development of replacements that offer better performance and are less likely to degrade automobile catalytic converters over time. Using atomic force microscopy in ZDDP-containing lubricant base stock at elevated temperatures, we monitored the growth and properties of the tribofilms in situ in well-defined single-asperity sliding nanocontacts. Surface-based nucleation, growth, and thickness saturation of patchy tribofilms were observed. The growth rate increased exponentially with either applied compressive stress or temperature, consistent with a thermally activated, stress-assisted reaction rate model. Although some models rely on the presence of iron to catalyze tribofilm growth, the films grew regardless of the presence of iron on either the tip or substrate, highlighting the critical role of stress and thermal activation.

Quantized friction across ionic liquid thin films

Ionic liquids - salts in the liquid state under ambient conditions - are of great interest as precision lubricants. Ionic liquids form layered structures at surfaces, yet it is not clear how this nano-structure relates to their lubrication properties. We measured the friction force between atomically smooth solid surfaces across ionic liquid films of controlled thickness in terms of the number of ion layers. Multiple friction-load regimes emerge, each corresponding to a different number of ion layers in the film. In contrast to molecular liquids, the friction coefficients differ for each layer due to their varying composition.

Switching Atomic Friction by Electrochemical Oxidation

Friction between the sliding tip of an atomic force microscope and a gold surface changes dramatically upon electrochemical oxidation of the gold surface. Atomic-scale variations of the lateral force reveal details of the friction mechanisms. Stick-slip motion with atomic periodicity on perfect Au(111) terraces exhibits extremely low friction and almost no dependence on load. Significant friction is observed only above a load threshold at which wear of the surface is initiated. In contrast, irregular stick-slip motion and a linear increase of friction with load are observed on electrochemically oxidized surfaces. The observations are discussed with reference to the amorphous structure of the oxo-hydroxide surface and atomic place exchange mechanisms upon oxidation. Reversible, fast switching between the two states of friction has been achieved in both perchloric and sulfuric acid solutions.

Solvation and squeeze out of hexadecane on graphite

We have performed simultaneous force and conductivity measurement of hexadecane liquid confined between a conducting atomic force microscope tip and a graphite surface. Both the current and the force data reveal discrete solvation layering of the hexadecane near the surface. We typically observe that the current does not vary with load in a simple way as the layer closest to the surface is compressed, but increases markedly prior to the expulsion of material from the tip-sample gap. We infer that even for a nanoscale asperity there is conformation change of the confined hexadecane under the tip apex prior to squeeze out of the molecules.

Molecular order and disorder in the frictional response of alkanethiol self-assembled monolayers.

Molecular processes in the frictional response of an alkanethiol monolayer, self-assembled on a Au(111) surface, are studied by means of high-resolution friction force microscopy in ultrahigh vacuum. With increasing load, three regimes are observed on defect-free domains of the monolayer: smooth sliding with negligible friction, regular molecular stick-slip motion with increasing friction, and the onset of wear in the monolayer. Molecular contrast in the lateral force is found for inequivalent molecules within the unit cell of the c(4 × 2) superstructure. Significant differences in the frictional response are found between defect-free domains and areas including a domain boundary. Friction increases by an order of magnitude on domain boundaries in connection with irregular stick-slip motion. This increased friction at domain boundaries is observed at loads below the onset of wear.

Friction, adhesion and wear durability of an ultra-thin perfluoropolyether-coated 3-glycidoxypropyltrimethoxy silane self-assembled monolayer on a Si surface

The tribological properties, such as coefficient of friction, adhesion and wear durability of an ultra-thin (<10 nm) dual-layer film on a silicon surface were investigated. The dual-layer film was prepared by dip-coating perfluoropolyether (PFPE), a liquid polymer lubricant, as the top layer onto a 3-glycidoxypropyltrimethoxy silane self-assembled monolayer (epoxy SAM)-coated Si substrate. PFPE contains hydroxyl groups at both ends of its backbone chain, while the SAM surface contains epoxy groups, which terminate at the surface. A combination of tests involving contact angle measurements, ellipsometry, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) was used to study the physical and chemical properties of the film. The coefficient of friction and wear durability of the film were investigated using a ball-on-disk tribometer (4 mm diameter Si3N4 ball as the counterface at a nominal contact pressure of ∼330 MPa). AFM was used to investigate the adhesion forces between a sharp Si3N4 tip and the film. This dual-layer film had a very low coefficient of friction, adhesion and wear when compared to epoxy SAM-coated Si only or bare Si surface. The reasons for the improved tribological performance are explained in terms of the lubrication characteristics of PFPE molecules, low surface energy of PFPE, covalent bonding between PFPE and epoxy SAM coupled with reduced mobile PFPE. The low adhesion forces coupled with high wear durability show that the film has applications as a wear resistant and anti-stiction film for microcomponents made from Si.

Liquid Atomic Force Microscopy: Solvation Forces, Molecular Order, and Squeeze-Out

We review the use of atomic force microscopy (AFM) in liquids to measure oscillatory solvation forces. We find solvation layering can occur for all the liquids studied (linear and branched alkanes) but marked variations in the force and dissipation may arise dependent on: a) the temperature, b) the tip shape/radius of curvature, and c) the degree of molecular branching. Several findings (e.g., the strong temperature dependence in measured solvation forces, solvation oscillations using branched molecules) differ from those observed using the Surface Force Apparatus, because of the nanoscale area probed by AFM. Conduction AFM is used to explore how liquid is squeezed out of the tip–sample gap, and enables the change in contact area of the tip–sample junction to be monitored and compared to mechanical models. We find elastic models provide a good description of the deformation of ordered, solid-like solvation layers but not disordered, liquid-like layers.

Direct torsional actuation of microcantilevers using magnetic excitation

Torsional mode dynamic force microscopy can be used for a wide range of studies including mapping lateral contact stiffness, torsional frequency or amplitude modulation imaging, and dynamic friction measurements of various materials. Piezo-actuation of the cantilever is commonly used, but it introduces spurious resonances, limiting the frequency range that can be sampled, and rendering the technique particularly difficult to apply in liquid medium where the cantilever oscillations are significantly damped. Here, we demonstrate a method that enables direct torsional actuation of cantilevers with high uniformity over wide frequency ranges by attaching a micrometer-scale magnetic bead on the back side of the cantilever. We show that when beads are magnetized along the width of the cantilever, efficient torsional actuation of the cantilevers can be achieved using a magnetic field produced from a solenoid placed underneath the sample. We demonstrate the capability of this technique by imaging atomic steps on graphite surfaces in tapping mode near the first torsional resonance of the cantilever in dodecane. The technique is also applied to map the variations in the lateral contact stiffness on the surface of graphite and polydiacetylene monolayers.

Enhanced quality factors and force sensitivity by attaching magnetic beads to cantilevers for atomic force microscopy in liquid

Dynamic-mode atomic force microscopy (AFM) in liquid remains complicated due to the strong viscous damping of the cantilever resonance. Here, we show that a high-quality resonance (Q>20) can be achieved in aqueous solution by attaching a microgram-bead at the end of the nanogram-cantilever. The resulting increase in cantilever mass causes the resonance frequency to drop significantly. However, the force sensitivity—as expressed via the minimum detectable force gradient—is hardly affected, because of the enhanced quality factor. Through the enhancement of the quality factor, the attached bead also reduces the relative importance of noise in the deflection detector. It can thus yield an improved signal-to-noise ratio when this detector noise is significant. We describe and analyze these effects for a set-up that includes magnetic actuation of the cantilevers and that can be easily implemented in any AFM system that is compatible with an inverted optical microscope.