Session U33: Polymers and Soft Solids at Interfaces: Tribology, Wear, Rheology and Interactions

Formation of Pickering Emulsions Using Nanodiamonds

Pickering emulsions are used over surfactant stabilized emulsions in a wide range of applications including personal care products, pharmaceuticals, and oil recovery because of their enhanced stability and lower toxicity. We examine here the use of nanodiamonds (ND), a relatively novel carbonaceous filler with high adsorption activity, small size, and large surface area to create solid stabilized emulsions. Using a system consisting of isopropyl palmitate and water, stabilized by hydroxylated NDs, we investigate the stability, rheology and frictional behavior of these emulsions as a function of ND concentration. Optical microscopy reveals increasing ND concentration results in smaller droplet sizes, due to the greater availability of particles that can be adsorbed on the oil-water interface. This behavior is consistent with our rheological results of higher G’ and yield stress with increased ND, as the presence of smaller droplets facilitate the formation of a densely packed network. Microstructure recovery after breakdown at different stress levels correlate with the ratio of applied stress to the yield stress for each ND concentration. Tribological behavior of ND emulsions was also investigated using a soft model contact and the data related to the rheological characteristics.   

Utilizing Inorganic Nanoadditives to Influence the Surface Properties of Polymer Films

Addition of nanosized inorganic materials to a polymer matrix results in nanohybrids with optimized properties. In this work, we report on the development of superhydrophobic and water repellent polymer coatings by utilizing nanoadditives of different geometries and size. The nanocomposite coatings were deposited either on a soft polyethylene, PE, substrate or on a hard silicon wafer. The coating morphology and effective roughness were studied with Scanning Electron Microscopy and profilometry, respectively, as a function of the nanoadditive content. In the case of PE substrates, the optical clarity of the original film was preserved following the nanohybrid coating, as verified by UV-Vis spectroscopy. The surface properties of all films were investigated by contact angle measurements; the water contact angle showed a non-monotonic dependence on the coating composition and depended strongly on the kind of nanoadditive whereas the contact angle hysteresis was significantly affected by its presence.   

Interfacial Dynamics of Confined Microgel Liquids on Soft Surfaces

While much research has strived to understanding the dynamics of confined liquids on a hard solid surface, the dynamics of liquids confined on a soft and deformable surface remains unclear. In this work, we employed poly(N-isopropylacrylamide) (PNIPAM) microgels of varied crosslinking density as the confined liquids as well as confining surface coatings. We analyzed the mean-squared-displacement (MSD) of confined PNIPAM microgel particles in the first 1-2 particle layers immediately adjacent to PNIPAM coated surface of varied particle-to-surface elasticity ratios. We observed that soft surface of low elasticity show little impact on the interfacial dynamics of confined microgel particles of varied elasticity, where the MSD of confined microgels exhibits little deviation from their bulk behaviors. Such interfacial behavior is in sharp contrast to the dynamic arrest of microgels confined on hard surface of high elasticity. Furthermore, we found that the dynamic heterogeneity in confined glass-like microgel liquids could diminish with decreasing the elasticity of confining surface, which gives insight to design low-friction and lubricious coatings and surfaces.
  

How osmotic pressure governs sliding and surface structures of swollen crosslinked hydrogels

High-water-content hydrogels are being increasingly explored for applications in biomedicine, water filtration, and hybrid materials. As these materials slide against hard, impermeable countersurfaces, they exhibit time-dependent and history-dependent friction related to their response under stress. Here I present two vignettes of the role of osmotic pressure in understanding hydrogel lubrication. The first considers the competitive rates of surface slip and pressure-driven dehydration due to applied loads. Given initial measurements of friction at very low and very high speeds, the ratio of the timescales of these effects can predict friction along the intermediate spectrum. The second vignette describes the inherent generation of less-dense surface layers (~single microns) that arise from the bulk due to the discontinuity of osmotic pressure between the bulk and the open bath submerging a crosslinked hydrogel. These layers are confirmed with multiscale indentation techniques and material creep localized to the near-surface region.   

Bio-inspired surface modification of PDMS to reduce dry friction

Reducing friction and associated energy dissipation in mechanical systems is critical for a broad range of applications. Under dry condition, soft elastomers are inherently adhesive and display high friction coefficient, which leads to severe wear and damage during contact. The skin structure and the scales geometry of some squamate reptiles, especially snakes and lizards, offers a potential way for friction reduction under extremely dry conditions. The stiff keratin-based epidermis on soft dermic tissue helps snakes reduce friction in forward locomotion. This study aims to mimic such functional skins by modifying both the stiffness and topography of the elastomeric surfaces. Using a photoinitiated infiltration polymerization, glassy polyHEMA is formed within the skin of PDMS, in the presence of water. The as-prepared surface displays a integrated skin layer possessing fold-like structure, with stiffness increasing from glassy to rubbery with the increase of load. Under dry condition, the friction coefficient of the PDMS reduces from 1.6 to 0.14 with the modification.
  

Friction and wear of polyzwitterionic brush-grafted surfaces

Inspired by the hydrophilic biomacromolecules adsorbed to the surface of articular cartilage, we investigate the tribology of betainized poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes grafted on soft poly(dimethyl siloxane) (PDMS) and hard silicon substrates. These polyzwitterions are known to provide excellent lubricity due to their ability to retain water in their side groups. Surfaces are hydrolyzed to introduce surface silanol groups which serve as an attachment point for a trichlorosilane initiator. After initiator deposition, PDMAEMA is grown from the surface via atom transfer radical polymerization (ATRP) and then betainized to render it zwitterionic. We measure the steady state and transient friction coefficients of the substrates as a function of brush film thickness and substrate elasticity. Our initial results suggest that use of soft, deformable substrates such as PDMS yields increased wear resistance for brush films due to a reduction in the contact pressure below the yield point of the polymer brushes. Furthermore, repetitive transient contact appears to result in increased wear resistance, with the total shearing time required for noticeable wear to occur increasing by an order of magnitude.   

Surface Forces Apparatus Measurements Between Oppositely Charged Polyelectrolyte Brushes as a Function of Ionic Strength

Interactions between polyanions and polycations result in polyelectrolyte complexation and the formation of polymer dilute (aqueous) and polymer rich (coacervate) phases. Understanding the forces governing the formation and stability of these complexes as well as their dependence on solution conditions is of great interest. We directly measure the interactions between densely grafted, apposing polycationic and polyanionic brushes as a function of ionic strength using the surface forces apparatus (SFA). Additionally, we compare these force profiles with SFA measurements between apposing polyanionic brushes and between apposing polycationic brushes.   

Tribology of soft colloidal microgels: An oral perspective

Oral tribology is emerging as a new paradigm in the tribology field to quantify friction in soft sliding contact surfaces¹. Using a combination of experimental techniques and theoretical considerations, this talk will cover three case studies^2-4 on tribology of soft elastomeric surfaces (with different wetting properties) in the presence of biopolymeric microgels with well-defined deformability, designed using variable cross-linking densities and particle sizes. Some of these microgels show aqueous ‘ball-bearing’ abilities depending upon their volume fraction². A case study⁴ will be presented on how these microgels can act as viscosity modifiers of the continuum, where the lubrication performance can be quantitatively described using the Newtonian plateau value (η_∞). Finally, ongoing research on development of novel soft tribo-surfaces to emulate the highly sophisticated oral mucosal surfaces engineered by the nature will be highlighted ¹.

References
1. Sarkar, et al., Curr. Opin. Colloid Interface Sci 39, 61-75 (2019).
2. Sarkar, et al., Langmuir 33, 14699-14708 (2017).
3. Torres, et al.,.Sarkar, ACS Appl. Mater. Interfaces 10, 26893-26905 (2018).
4. Sarkar, et al., Soft Matter Accepted (2019).   

Pore-size dependence and glassy behavior of hydrogel friction on smooth surfaces

Hydrogels are important in many scientific and engineering applications due to their tunable physiochemical properties and bio-compatibility. Bulk hydrogels consist of a crosslinked polymer matrix imbibed with water or a similar solvent at volume fractions that can exceed 90%, leading to a rich spectrum of interfacial rheological behaviors. Using a custom-built, continuous pin-on-disc tribometer, here we identify three distinct regimes of frictional behavior for both polyacrylamide (PAAm) and agarose hydrogel spheres on smooth surfaces. At low velocities, friction is controlled by hydrodynamic flow through the porous hydrogel network, and is inversely proportional to the characteristic pore size. At high velocities, a mesoscopic, lubricating liquid film forms obeying elastohydrodynamic theory. In between these regimes, the frictional force sharply decreases with velocity and simultaneously displays time-dependent behavior characteristic of glassy, slow relaxation over several minutes. This relaxation strongly depends on the fluid shear rate, and the transition can be tuned by varying the solvent salt concentration, solvent viscosity, and sliding geometry at the interface.   

Study of the tribological behavior of hydrogel-like materials with an extended Surface Forces Apparatus

Biological tribosystems are excellent examples of nature leveraging material properties to achieve exceptional lubrication for prolonged periods of activity. In these systems, lubrication is provided by hydrogel-like surface layers and an aqueous lubricant. Our research aims to advance the knowledge about the relation between frictional dissipation and hydrogel’s microstructure. In this talk, we will present steady and dynamic shear measurements with an extended surface forces apparatus (eSFA) on hydrogel thin films under modulated compression. Complimentary quasi-equilibrium compression measurements with the eSFA can be performed during tribological measurements; this is a unique capability of the SFA and enables to detect polymer rearrangements upon shear. This type of experiments provides new insight into the influence of hydrogel’s microstructure on dissipative mechanisms that dictate lubrication with special emphasis on the influence of hydrogel’s microstructure.   

Indentation of a microparticle into an oil-coated, soft silicone surface

Small scale contact between a soft, liquid-coated layer and a stiff surface is found in many situations, from synovial fluid on articular cartilage to adhesives in humid environments. Moreover, many model studies on soft adhesive contacts are conducted with soft silicone elastomers, which usually possess uncrosslinked molecules (i.e. liquid silicone oil) when the modulus is low. The presence of liquid near the contact line can cause capillary forces on the particle. We consider a similar situation in which uncrosslinked liquid molecules are already placed at a surface prior to contact. More specifically, we investigate how the thickness of an oil layer relates to the indentation depth of a glass microsphere into a lightly crosslinked PDMS network. A simple model that balances the capillary force of the oil layer with the elastic force from the substrate is proposed to predict the position of the particle. Interestingly, the vertical force associated with a thin layer of liquid on a soft substrate appears to govern the indentation depth of a microsphere into the polymer network.