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We present a novel approach to probe elastic properties of polyelectrolyte multilayer microcapsules. The method is based on measurements of the capsule load-deformation curves with the atomic force microscope. The experiment suggests that at low applied load deformations of the capsule shell are elastic. Using elastic theory of membranes we relate force, deformation, elastic moduli, and characteristic sizes of the capsule. Fitting to the prediction of the model yields the lower limit for Young's modulus of the polyelectrolyte multilayers of the order of 1 − 100 MPa, depending on the template and solvent used for its dissolution. These values correspond to Young's modulus of an elastomer.

We study the deformation of nonadhesive polyelectrolyte microcapsules under applied load using an atomic force microscope (AFM)-related force measuring device. Both "hollow" (water inside) and "filled" (waterpolyanion solution inside) microcapsules are explored. The "filled" capsules were found to be much stiffer than "hollow" ones. The load-deformation profiles always included two regimes, characterized by different behavior. In the first regime, with a low applied load, capsule deformation is elastic and reversible. Above a certain load, capsules deform substantially and partly irreversibly. In this regime, the "hollow" capsules show variability in the reversibility, as well as in load-deformation profiles, which include different sectors (from substantial deformation at quasiconstant load to noisy regions). The "filled" capsules do not reveal such variability and become stiffer when the load is increased. After substantial deformation the "hollow" capsules enter a third region, in which major damage is caused by higher load. We show that the dramatic changes of the capsule's mechanical properties after filling with polyelectrolyte reflect a combined effect of excess osmotic pressure inside them, changes in the shell stiffness, and possibly a formation inside capsules of an electrostatically stabilized 3D net structure.

We describe a tensorial generalization of the Navier slip boundary condition and illustrate its use in solving for flows around anisotropic textured surfaces. Tensorial slip can be derived from molecular or microstructural theories or simply postulated as an constitutive relation, subject to certain general constraints on the interfacial mobility. The power of the tensor formalism is to capture complicated effects of surface anisotropy, while preserving a simple fluid domain. This is demonstrated by exact solutions for laminar shear flow and pressure-driven flow between parallel plates of arbitrary and different textures. From such solutions, the effects of rotating a texture follow from simple matrix algebra. Our results may be useful to extracting local slip tensors from global measurements, such as the permeability of a textured channel or the force required to move a patterned surface, in experiments or simulations.

Abstract. We discuss how the wettability and roughness of a solid impacts its hydrodynamic properties. We see in particular that hydrophobic slippage can be dramatically affected by the presence of roughness. Owing to the development of refined methods for setting very well-controlled micro-or nanotextures on a solid, these effects are being exploited to induce novel hydrodynamic properties, such as giant interfacial slip, superfluidity, mixing, and low hydrodynamic drag, that could not be achieved without roughness.

We present the results of investigations of high-speed drainage of a thin film confined between a microscopic colloidal probe and a substrate performed with a new atomic force microscope-related setup. Theoretical calculations are used to formulate the governing equation (force balance) for instantaneous deflection of a cantilever spring, which is due to both concentrated forces acting on a colloidal probe and viscous drag force on a cantilever itself. The suggested way to subtract the latter contribution allows design of a lubrication experiment. Two pairs of interacting solids, characterized by different wettability and smoothness, immersed into water-electrolyte solutions have been studied. Results for hydrophilic silica surfaces are in excellent agreement with the Reynolds theory of hydrodynamic lubrication. Faster drainage of a thin film confined between hydrophobic rough polystyrene surfaces is consistent with the theory of film drainage between slippery surfaces. The slip lengths are found to be of the order of the size of asperities, and do not depend on the separation and shear rate. The results are important for colloidal dynamics and nanofluidics.

We report results of investigations of a high-speed drainage of thin aqueous films squeezed between randomly nanorough surfaces. A significant decrease in the hydrodynamic resistance force as compared with that predicted by Taylor's equation is observed. However, this reduction in force does not represent the slippage. The measured force is exactly the same as that between equivalent smooth surfaces obeying no-slip boundary conditions, but located at the intermediate position between peaks and valleys of asperities. The shift in hydrodynamic thickness is shown to be independent of the separation and/or shear rate. Our results disagree with previous literature data reporting very large and shear-dependent boundary slip for similar systems.