The rolling of a cylinder on a flat plate can be viewed as the propagation of two crackssone closing at the advancing edge and the other opening at the trailing edge. The difference of adhesion in these two regions, i.e. the adhesion hysteresis, depends on the nonequilibrium interfacial processes in an elastic system. This rolling contact geometry was used to study the effects of dispersion forces and specific interactions on interfacial adhesion hysteresis. In order to accomplish this objective, hemicylindrical elastomers of polydimethylsiloxane (PDMS)sboth unmodified and plasma oxidizedswere rolled on thin PDMS films bonded to silicon wafers. Plasma oxidation generates a silica-like surface on PDMS elastomer, which interacts with PDMS molecules via hydrogen-bonding forces. The adhesion hysteresis in the latter case is large and depends significantly on the molecular weight of the grafted polymer, whereas the hysteresis is rather negligible for purely dispersive systems. These results are interpreted in terms of the orientation and relaxation of polymer chains, which has its origin in the Lake-Thomas effect.
An atomic force microscope tip, coated with a small amount of liquid silicone, was used to investigate the wetting and capillary bridging forces on various low-and high-energy surfaces. The low-energy surfaces were prepared by reacting alkyl and perfluoroalkyl functional silanes with a silicon wafer (Si/SiO2). Forcedistance scans in air revealed that the silicone fluid forms ductile capillary bridges on the low-energy methyl and perfluoromethyl surfaces, whereas a tight bridge is formed on silica. Further studies on a silicon wafer possessing a gradient of surface energy shed more light on the relationship between surface wettability and capillary forces. These observations can be modeled in a general way using the Young-Laplace equation. The understanding of these capillary interactions at nanoscopic levels may have important applications, especially in the controlled deposition of liquid droplets on surfaces.
The mechanical dispersion technology used in this study employs rotor-stator mixers that produce water-continuous high internal phase emulsions (HIPEs) with narrow drop size distributions and small drop sizes, even when the internal phase (oil) viscosity is quite high. Analysis of these HIPEs reveals trends that are consistent with formation by a capillary instability mechanism in which a shear deformation produces highly elongated drops that rupture to form uniform, small droplets. In the search for a predictive tool to aid in the manufacture and use of HIPEs, rheology data for these shear-thinning HIPEs have been compared to data for models in the literature. Existing models do not correctly account for the effect of a high internal phase viscosity on the rheological properties of the HIPE. Another shortcoming is failure to correctly address the shear-thinning exponent. Whereas internal phase viscosity does not seem to affect the shear-thinning exponent, the surfactant apparently plays an important role, possibly through its modification of the interfacial tension and continuous phase rheology.
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