Shivaprakash N. Ramakrishna

Controlling Adhesion Force by Means of Nanoscale Surface Roughness

Control of adhesion is a crucial aspect in the design of microelectromechanical and nanoelectromechanical devices. To understand the dependence of adhesion on nanometer-scale surface roughness, a roughness gradient has been employed. Monomodal roughness gradients were fabricated by means of silica nanoparticles (diameter ∼12 nm) to produce substrates with varying nanoparticle density. Pull-off force measurements on the gradients were performed using (polyethylene) colloidal-probe microscopy under perfluorodecalin, in order to restrict interactions to van der Waals forces. The influence of normal load on pull-off forces was studied and the measured forces compared with existing Hamaker-approximation-based models. We observe that adhesion force reaches a minimum value at an optimum particle density on the gradient sample, where the mean particle spacing becomes comparable with the diameter of the contact area with the polyethylene sphere. We also observe that the effect on adhesion of increasing the normal load depends on the roughness of the surface.

Poly(acrylamide) films at the solvent-induced glass transition: adhesion, tribology, and the influence of crosslinking

Adhesive and nanotribological properties of end-grafted poly(acrylamide) (PAAm) films with various degrees of crosslinking, and in the presence of solvents over a broad spectrum of quality, were investigated by means of colloidal-probe atomic force microscopy. The solvent consisted of a mixture of water (good solvent for PAAm) and methanol (bad solvent for PAAm). Adhesion measurements carried out on brush (uncrosslinked) structures revealed significant pull-off forces in solvent mixtures that placed the polymer at its glass transition. These pull-off forces, which were orders of magnitude higher than those measured in either pure solvent, were significantly reduced in the presence of crosslinking. The nanostructures of PAAm films with different crosslinking degrees were elucidated in their maximum collapsed state by means of atomic force microscopy, and the parameters influencing adhesive properties of films with brush structures at their glass transition were investigated. Complex nanotribological behavior of PAAm films was observed, and found to result from the interplay of film structure and adhesion forces, which were influenced by both crosslinking degree and solvent quality.

Study of adhesion and friction properties on a nanoparticle gradient surface: transition from JKR to DMT contact mechanics.

We have previously investigated the dependence of adhesion on nanometer-scale surface roughness by employing a roughness gradient. In this study, we correlate the obtained adhesion forces on nanometer-scale rough surfaces to their frictional properties. A roughness gradient with varying silica particle (diameter ≈ 12 nm) density was prepared, and adhesion and frictional forces were measured across the gradient surface in perfluorodecalin by means of atomic force microscopy with a polyethylene colloidal probe. Similarly to the pull-off measurements, the frictional forces initially showed a reduction with decreasing particle density and later an abrupt increase as the colloidal sphere began to touch the flat substrate beneath, at very low particle densities. The friction-load relation is found to depend on the real contact area (A(real)) between the colloid probe and the underlying particles. At high particle density, the colloidal sphere undergoes large deformations over several nanoparticles, and the contact adhesion (JKR type) dominates the frictional response. However, at low particle density (before the colloidal probe is in contact with the underlying surface), the colloidal sphere is suspended by a few particles only, resulting in local deformations of the colloid sphere, with the frictional response to the applied load being dominated by long-range, noncontact (DMT-type) interactions with the substrate beneath.

Adhesion and Friction Properties of Polymer Brushes on Rough Surfaces: A Gradient Approach

The effect of nanoscale surface roughness on the lubrication properties of a polymer brush in a good solvent has been investigated. Friction and adhesion forces were measured by means of polyethylene colloidal-probe AFM across a 12 nm silica particle gradient before and after the adsorption of a poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) polymer brush. The adsorption and conformation of the polymer chains were studied with multiple transmission and reflection infrared (MTR-IR) spectroscopy. The results show that prior to the adsorption of PLL-g-PEG on the gradient surface, the friction is high at the smooth end of the gradient while it decreases toward the rough end. Moreover, there is a direct correlation between friction and adhesion. Upon adsorption of the brushes, adhesion vanishes. In this case, a higher frictional force between the PEG-coated particle gradient substrate and the polyethylene sphere is observed at the rough end of the gradient in comparison to the smooth end. In spite of the increased adsorbed mass of PLL-g-PEG at the rough end of the gradient, theory and simulations show that the high curvature of the nanoparticles leads to a less swollen PEG brush in comparison to PEG brushes adsorbed on a planar surface, resulting in a lower repulsion, which can explain the observed increase in friction with particle density.

Exploring Lubrication Regimes at the Nanoscale: Nanotribological Characterization of Silica and Polymer Brushes in Viscous Solvents

Nanotribological properties of silica surfaces, with and without adsorbed, brushlike copolymers of poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) and poly(L-lysine)-graft-dextran (PLL-g-dextran) have been investigated in aqueous viscous solvent mixtures by means of colloid-probe lateral force microscopy. Lateral forces for PEG/dextran brushes have been measured as a function of shear velocity in aqueous mixtures of glycerol and ethylene glycol (EG), which are highly miscible with water, but are poor solvents for hydrophilic PEG and dextran chains. Prior to the friction measurements on polymer brushes, a nanoscale Stribeck curve was obtained on a bare silica surface in the selected aqueous cosolvent mixtures. The Stribeck curve for bare surfaces indicates the existence of a surface-solvating thin film due to the adsorption of hydrated ions, preventing direct silica-silica contact in the boundary-lubrication regime. A clear transition to the hydrodynamic regime is seen at high speeds for solvents with higher viscosities. The polymer brushes, however, show a shear-thinning effect with increasing shear speed and a combined influence of polymer film and solvent viscosity on the measured friction forces. The formation of an interfacial fluid-film is shown to shift the hydrodynamic regime of hydrated brushes to a lower value of Uη. The correlation between the structural configuration and the corresponding frictional properties of the polymer brushes upon changing solvent quality is discussed.

Next-Generation Polymer Shells for Inorganic Nanoparticles are Highly Compact, Ultra-Dense, and Long-Lasting Cyclic Brushes

Cyclic poly-2-ethyl-2-oxazoline (PEOXA) ligands for superparamagnetic Fe3 O4 nanoparticles (NPs) generate ultra-dense and highly compact shells, providing enhanced colloidal stability and bio-inertness in physiological media. When linear brush shells fail in providing colloidal stabilization to NPs, the cyclic ones assure long lasting dispersions. While the thermally induced dehydration of linear PEOXA shells cause irreversible aggregation of the NPs, the collapse and subsequent rehydration of similarly grafted cyclic brushes allow the full recovery of individually dispersed NPs. Although linear ligands are densely grafted onto Fe3 O4 cores, a small plasma protein such as bovine serum albumin (BSA) still physisorbs within their shells. In contrast, the impenetrable entropic shield provided by cyclic brushes efficiently prevents nonspecific interaction with proteins.

Layering of ionic liquids on rough surfaces.

Understanding the behavior of ionic liquids (ILs) either confined between rough surfaces or in rough nanoscale pores is of great relevance to extend studies performed on ideally flat surfaces to real applications. In this work we have performed an extensive investigation of the structural forces between two surfaces with well-defined roughness (<9 nm RMS) in 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide by atomic force microscopy. Statistical studies of the measured layer thicknesses, layering force, and layering frequency reveal the ordered structure of the rough IL-solid interface. Our work shows that the equilibrium structure of the interfacial IL strongly depends on the topography of the contact.

Tuning surface mechanical properties by amplified polyelectrolyte self-assembly: where "grafting-from" meets "grafting-to".

We report the interaction of surface-tethered weak polyacid brushes, poly(methacrylic acid), with a weak polybase poly(L-lysine)-graft-poly(ethylene glycol), in solution. The grafted polyacid brushes, grown directly from the silicon substrate by UVLED surface-initiated polymerization, act as a nanotemplate for the solution-phase polybase, which penetrates into the brushes, forming a polyelectrolyte complex (PEC), whose mechanical and nanotribological properties are markedly influenced by the electrostatic assembly conditions. The mechanical effects are amplified due to the architecture of the specific polybase used, which contributes approximately 2k Da per unit charge to the overall system, resulting in an efficient filling of the polyacid brushes, which thus acts as a scaffold. The distribution of the adsorbed copolymers in the PEC films has been investigated by means of confocal microscopy. The unique structure of the PEC films provides a system whose mechanical and nanotribological properties can be tuned over a wide range.

Roughness-dependent tribology effects on discontinuous shear thickening

Significance Shear thickening is a ubiquitous rheological phenomenon whereby dense suspensions of particles in a fluid exhibit a viscosity increase at high shear, which can turn into a viscosity divergence [discontinuous shear thickening (DST)]. Although macroscopically well characterized, the microscopic origin of DST is still debated, especially in connection to particle surface properties, e.g., roughness and friction. We elucidate here the mechanisms underpinning DST by carrying out nanotribological measurements of the interparticle contacts of model rough colloids. We demonstrate that rough particles exhibit DST over a broader range of shear rates and for volume fractions much lower than for smooth colloids, due to interlocking of surface asperities, showing that taking an engineering-tribology approach is a powerful way to tune DST. Surface roughness affects many properties of colloids, from depletion and capillary interactions to their dispersibility and use as emulsion stabilizers. It also impacts particle–particle frictional contacts, which have recently emerged as being responsible for the discontinuous shear thickening (DST) of dense suspensions. Tribological properties of these contacts have been rarely experimentally accessed, especially for nonspherical particles. Here, we systematically tackle the effect of nanoscale surface roughness by producing a library of all-silica, raspberry-like colloids and linking their rheology to their tribology. Rougher surfaces lead to a significant anticipation of DST onset, in terms of both shear rate and solid loading. Strikingly, they also eliminate continuous thickening. DST is here due to the interlocking of asperities, which we have identified as “stick–slip” frictional contacts by measuring the sliding of the same particles via lateral force microscopy (LFM). Direct measurements of particle–particle friction therefore highlight the value of an engineering-tribology approach to tuning the thickening of suspensions.

Design and characterization of ultrastable, biopassive and lubricious cyclic poly(2-alkyl-2-oxazoline) brushes

Bilayer polymer brushes presenting surface-bound poly(glycidyl methacrylate) (PGMA) films and interfacial cyclic poly(2-alkyl-2-oxazoline) (PAOXA) brushes show excellent biopassivity and lubrication, while displaying long-term stability in chemically harsh aqueous environments. Due to their lower radii of gyration (Rg), cyclic poly(2-methyl-2-oxazoline)s (PMOXAs) and poly(2-ethyl-2-oxazoline)s (PEOXAs) react at high temperatures with PGMA grafts producing ∼50% denser brushes compared to linear analogues featuring comparable molar masses. This generates significantly more hydrated brush interfaces, which quantitatively prevent unspecific surface contamination by biomolecules after several hours of exposure. In addition, the more compact and denser character of cyclic brushes imparts excellent lubricating properties to the bilayered coatings, with the more hydrophilic cyclic PMOXA interfaces reaching a coefficient of friction (μ) of 0.05 against a silica AFM probe in aqueous medium. In addition to their unique physicochemical properties, cyclic PMOXA and PEOXA brushes grafted on PGMA layers demonstrate extremely robust films, which could withstand one month incubation in phosphate buffered saline (PBS) solution, tap water or water from Lake Zurich.