http://experimentationlab.berkeley.edu/sites/default/files/AFMImages/butt_AFM.pdf

Force measurements with the atomic force microscope: Technique, interpretation and applications

Differently from passive Brownian particles, active particles, also known as self-propelled Brownian particles or microswimmers and nanoswimmers, are capable of taking up energy from their environment and converting it into directed motion. Because of this constant flow of energy, their behavior can be explained and understood only within the framework of nonequilibrium physics. In the biological realm, many cells perform directed motion, for example, as a way to browse for nutrients or to avoid toxins. Inspired by these motile microorganisms, researchers have been developing artificial particles that feature similar swimming behaviors based on different mechanisms. These man-made micromachines and nanomachines hold a great potential as autonomous agents for health care, sustainability, and security applications. With a focus on the basic physical features of the interactions of self-propelled Brownian particles with a crowded and complex environment, this comprehensive review will provide a guided tour through its basic principles, the development of artificial self-propelling microparticles and nanoparticles, and their application to the study of nonequilibrium phenomena, as well as the open challenges that the field is currently facing.

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Active Particles in Complex and Crowded Environments

Within the field of nanotechnology, nanoparticles are one of the most prominent and promising candidates for technological applications. Self-assembly of nanoparticles has been identified as an important process where the building blocks spontaneously organize into ordered structures by thermodynamic and other constraints. However, in order to successfully exploit nanoparticle self-assembly in technological applications and to ensure efficient scale-up, a high level of direction and control is required. The present review critically investigates to what extent self-assembly can be directed, enhanced, or controlled by either changing the energy or entropy landscapes, using templates or applying external fields.

Directed Self-Assembly of Nanoparticles

1: Magnetic Particles: Preparation, Properties and Applications: M. Ozaki. 2: Maghemite (gamma-Fe2O3): A Versatile Magnetic Colloidal Material C.J. Serna, M.P. Morales. 3: Dynamics of Adsorption and Oxidation of Organic Molecules on Illuminated Titanium Dioxide Particles Immersed in Water M.A. Blesa, R.J. Candal, S.A. Bilmes. 4: Colloidal Aggregation in Two-Dimensions A. Moncho-Jorda, F. Martinez-Lopez, M.A. Cabrerizo-Vilchez, R. Hidalgo Alvarez, M. Quesada-PMerez. 5: Kinetics of Particle and Protein Adsorption Z. Adamczyk.

Surface and Colloid Science

In a historical context the interface between two phases has played only a minor role in the physics of fluid dynamics. It is of course true that boundary conditions at interfaces, usually imposed as continuity of ve­ locity and stress, determine the velocity field of a given flow; however, this is a more or less passive use of the interface that allows one to ignore the structure of the transition between two phases. When an interface has been assigned a more active role in flow processes, it generally has been assumed that one parameter, the interfacial (surface) tension, accounts for all mech­ anical phenomena (Young et al. 1 959, Levich & Krylov 1969). In these studies, kinematic effects of the interface were not considered, and the "no-slip" condition on the velocity at interfaces was retained. The basic message of this article is that the interface is a region of small but finite thickness, and that dynamical processes occurring within this region lead not only to interfacial stresses but also to an apparent "slip velocity" that, on a macroscopic length scale, appears to be a violation of the no-slip condition. The existence of a slip velocity at solid/fluid interfaces opens a class of flow problems not generally recognized by the fluid-dynamics community. Three previous articles in this series deal with flow caused by interactions between interfaces and external fields such as electrical potential, tem­ perature, and solute concentration. Melcher & Taylor ( 1969) and Levich & Krylov (1969) consider fluid/fluid interfaces where stresses produced at the interface by the external field dictate the flow. Saville ( 1977), on the other hand, discusses the action of an electric field on a charged solid/fluid interface and reviews the currently accepted model for electrophoretic

Colloid Transport by Interfacial Forces

The rate of collection of Brownian particles under the influence of interaction forces between the collector surface and the particles is calculated by (a) incorporating the interaction forces in the rate constant of a virtual, first order, chemical reaction taking place on the surface of the collector, and by (b) solving the convective diffusion equation subject to that chemical reaction as a boundary condition. Several geometries (sphere, cylinder, rotating disc) are considered for the collector.An Arrhenius type equation is obtained for the apparent reaction rate constant. Equations for the apparent activation energy and for the frequency factor are established as functions of Hamaker's constant, ionic strength, surface potentials and particle radius.

Rate of deposition of Brownian particles under the action of London and double-layer forces

La microscopie a reflexion totale interne est une nouvelle technique utilisee pour mesurer directement le potentiel moyen de l'interaction entre une sphere microscopique et une plaque en verre

Total internal reflection microscopy: a quantitative tool for the measurement of colloidal forces

Simplified predictions of hamaker constants from Lifshitz theory

Numerical predictions of the migration velocity are presented for slightly non-uniform aqueous solutions of KCl or NaCl over a wide range of zeta potential (ζ) and particle radius divided by Debye length (κa). The characteristic speed of migration is comparable in magnitude to the ensemble mean diffusion velocity of ions forming the concentration gradient. In the absence of a macroscopic electric field induced by the gradient, the particle usually migrates toward higher salt concentration (the chemiphoretic contribution). However, for moderate values of κa, the particle reverses direction when |ζ| becomes sufficiently large. With an induced electric field, competition between electrophoresis and chemiphoresis can result in as many as four reversals in direction over the range of ζ. In some cases, the direction of migration was found to depend on the magnitude of the ionic drag coefficients, which seems to preclude predicting the direction on the basis of equilibrium thermodynamics.

Diffusiophoresis of a rigid sphere through a viscous electrolyte solution

A ray-optics scattering model has been developed to determine if multiple reflections between the sphere and the plate could alter the exponential relationship between the scattering intensity and the separation distance as contact is approached. Results indicate that the effect of multiple reflections is dependent on sphere size, refractive indices, and the penetration depth of the evanescent wave. An experimental validation of the model was performed with polystyrene spheres (diameters 7–30 μm) immersed in an alcohol mixture and resting on an MgF2 film that had the same refractive index. Film thicknesses varied between 0 and 300 nm. No significant effect of multiple reflections was measured at an incident angle approximately 2° above the critical angle, which was in agreement with the predictions of the ray-optics model. By contrast, the scattering intensity from a 300-μm sphere was predicted to be much more sensitive to the separation distance at separations below one penetration depth when the incident angle was increased to over 6° above the critical angle.

Dennis C. Prieve

Papers

Rate of deposition of Brownian particles under the action of London and double-layer forces

Total internal reflection microscopy: a quantitative tool for the measurement of colloidal forces

Simplified predictions of hamaker constants from Lifshitz theory

Diffusiophoresis of a rigid sphere through a viscous electrolyte solution

Scattering of an evanescent surface wave by a microscopic dielectric sphere.