Surface Roughness and Effective Stick-Slip Motion

The effect of random surface roughness on hydrodynamics of viscous incompressible liquid is discussed. Roughness-driven contributions to hydrodynamic flows, energy dissipation, and friction force are calculated in a wide range of parameters. When the hydrodynamic decay length (the viscous wave penetration depth) is larger than the size of random surface inhomogeneities, it is possible to replace a random rough surface by effective stick-slip boundary conditions on a flat surface with two constants: the stick-slip length and the renormalization of viscosity near the boundary. The stick-slip length and the renormalization coefficient are expressed explicitly via the correlation function of random surface inhomogeneities. The effective stick-slip length is always negative signifying the effective slow-down of the hydrodynamic flows by the rough surface (stick rather than slip motion). A simple hydrodynamic model is presented as an illustration of these general hydrodynamic results. The effective boundary parameters are analyzed numerically for Gaussian, power-law and exponentially decaying correlators with various indices. The maximum on the frequency dependence of the dissipation allows one to extract the correlation radius (characteristic size) of the surface inhomogeneities directly from, for example, experiments with torsional quartz oscillators.Comment: RevTeX4, 14 pages, 3 figure

Immobilized proteins in buffer imaged at molecular resolution by atomic force microscopy

Samples of supported planar lipid-protein membranes and actin filaments on mica were imaged by atomic force microscopy (AFM). The samples were fully submerged in buffer at room temperature during imaging. Individual proteins bound to the reconstituted membrane were distinguishable; some structural details could be resolved. Also, surface-induced, self-assembling of actin filaments on mica could be observed. Monomeric subunits were imaged on individual actin filaments. The filaments could be manipulated on or removed from the surface by the tip of the AFM. The process of the decoupling of the filamentous network from the surface upon changing the ionic conditions was imaged in real time

Surface characterization using atomic force microscopy (AFM) in liquid environments

Liquid imaging provides intrinsic advantages for AFM experiments, particularly for conducting in situ studies of chemical or biochemical reactions. Using liquid media has benefits for improving resolution, since the amount of force applied between the tip and sample can be reduced. Surface changes caused by immersion in different liquids can be investigated, such as for studying electrochemical reactions with different parameters of solvent polarity, pH or ion concentration. Aqueous buffers enable studies of biochemical reactions that simulate physiological conditions, with time-lapse capture of image frames at different intervals. Studies of surface changes throughout the course of self-assembly reactions have been monitored with AFM in liquid media. By injecting new molecules into the sample cell, AFM-based nanofabrication can be accomplished by nanografting protocols. Liquid environments expand the capabilities for scanning probe studies to provide insight for dynamic processes at the molecular-level. © Springer-Verlag Berlin Heidelberg 2013