This article demonstrates how the adhesion rates of micrometer-scale particles on a planar surface can be manipulated by nanometer-scale features on the latter. Here, approximately 500-nm-diameter spherical silica particles carrying a substantial and relatively uniform negative charge experienced competing attractions and repulsions as they approached and attempted to adhere to a negative planar silica surface carrying flat 11-nm-diameter patches of concentrated positive charge. The average spacing of these patches profoundly influenced the particle adhesion. For dense positive patch spacing on the planar collector, the particle adhesion was rapid, and the fundamental adhesion kinetics were masked by particle transport to the interface. For patch densities corresponding to a planar surface with net zero charge, particle adhesion was still rapid. Adhesion kinetics were observably reduced for patch spacings exceeding 20 nm and become slower with increased patch spacing. Ultimately, above a critical or threshold average patch spacing of 32 nm, no particle adhesion occurred. The presence of the threshold average patch spacing suggests that more than one positive surface patch was needed for particle capture under the particular conditions of this study. Furthermore, at the threshold, the length scales of the patch spacing and of the interactive surface area between the particle and the surface become similar: The concept of adhesion dominated by the matching of length scales is reminiscent of pattern recognition, even though the patch distribution on the collector is random in this work. Indeed, fluctuations play a critical role in these adhesion dynamics, hence the current behavior cannot be predicted by a mean field approach.

Using a model system based on electrostatics, we probe interactions between spherical particles (negative silica) and planar surfaces that present randomly placed discrete attractive regions, 10 nm in size, in a repulsive background (silica flats carrying cationic surface constructs). Experiments measure the adhesion rates of particles onto the patchy collecting surfaces from flowing dispersions, as a function of the surface loading of the attractive patches, for different particle sizes (0.5 and 1 mum diameter spheres) and different ionic strengths. Surfaces densely populated with patches, such that they present net electrostatic attractions to approaching particles, capture particles at the transport-limited (maximum) rate. Surfaces sparsely loaded with attractive patches (which present a repulsive mean field to approaching particles) are usually still adhesive, but the particle adhesion rate depends on particle size, ionic strength, and patch loading. Most significant is an adhesion threshold, a critical density of patches needed to capture particles. This threshold, which occurs at average patch spacings of 30 nm and larger and which can be tuned through ionic strength, comprises the ability of the patchy surfaces to selectively distinguish particles of different sizes or objects of different local curvature or roughness. The observation of such an adhesion threshold implicates spatial fluctuations in patch arrangement. In addition to experiments, this paper develops arguments for lengthscales that govern adhesion rate behavior, comparing particle geometry and fluctuation lengthscales, and then demonstrating qualitative consistency with the localized colloidal potentials involved.

Using quantitative phase microscopy, we have discovered a quadratic relationship between the radius R and the thickness t of helical ribbons that form spontaneously in multicomponent cholesterol-surfactant mixtures. These helical ribbons may serve as mesoscopic springs to measure or to exert forces on nanoscale biological objects. The spring constants of these helices depend on their submicroscopic thickness. The quadratic relationship (R ؔ t 2 ) between radius and thickness is a consequence of the crystal structure of the ribbons and enables a determination of the spring constant of any of our helices solely in terms of its observable geometrical dimensions.biological force spectroscopy ͉ elasticity of thin films ͉ phase-contrast microscopy in biophysics T he elastic properties of meso-and nanoscale thin elastic strips forming helical ribbons or tubules, have been the focus of active recent research in both biophysics and nanoscience communities (1-7). We have discovered that in a number of complex aqueous solutions containing a sterol (cholesterol in particular) and a mixture of surfactants, the sterol molecules may self-assemble into ribbons of helical shape (8). The geometry of the helical ribbons is characterized by the radius, width, thickness, contour length, and pitch angle, see figure 1a in ref. 9. Remarkably, the pitch angle is always either 11°or 54°, whereas axial length, width, and radius vary by two orders of magnitude in the range from 1 to Ϸ100 m. These helical ribbons are fascinating objects for fundamental studies (2,(8)(9)(10). Furthermore, because low-pitch helical ribbons have spring constants in the range of 0.5 to 500 pN/m (2), and the elongation of these springs from 1 m up to 100 m can easily be observed microscopically, it follows that they can be used as mesoscopic spring scales to measure forces between nanoscale biological objects in the range from 0.5 pN to 50 nN. For this and other applications, the ability to readily determine the spring constants of individual helixes is of crucial importance. In this article, we establish the relationship between the spring constant of the low-pitch cholesterol helical ribbons and its readily observable dimensions: width, radius, and length.Originally, it had been thought that cholesterol helical ribbons formed in surfactant mixtures had liquid crystalline structure and that their shape was governed by elastic properties of liquid crystalline layer (9,11,12). Recently, we have shown by X-ray diffraction that these helical ribbons are, in fact, single crystals with structure closely resembling that of cholesterol monohydrate (10). Having in mind the single-crystal nature of our ribbons, we have proposed that their helical shape is determined by a balance between two terms in the free energy of deformation of the cholesterol crystalline strip (2). The first term, the spontaneous bending energy, favors curling toward one of the two faces of the ribbon and is linear in curvature, ϪK s /R. The second term is the elastic energy of bending a strip. Thi...

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The hitherto unknown title compound is isolated from the system Na2SeO4‐H2SeO4‐H2O. Its space group is P1 with Z=1.