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.

/pdf/active-particles-in-complex-and-crowded-environments-4pyirdi1aj.pdf

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 surface potential of and double-layer interaction force between surfaces characterized by multiple ionizable groups.

Total-internal-reflection microscopy (TIRM) was used to monitor Brownian fluctuations in separation between a 10 µm polystyrene sphere and a glass plate when the two bodies are separated by a fraction of a micrometre of aqueous solution. Preliminary results clearly show evidence of double-layer repulsion which is weakened by increasing the ionic strength. Potential-energy profiles were obtained with a resolution of 2.5 nm. Although this falls short of the atomic resolution obtained with the crossed-mica cylinder technique for measuring colloidal forces, TIRM allows one of the interacting bodies to have colloidal dimensions. Besides measurement of colloidal and hydrodynamic forces, other applications include the study of hindered diffusion of the particle near a flat boundary and reversible adsorption of a single particle onto a plane surface.

Brownian motion of a hydrosol particle in a colloidal force field

The Dielectric Function for Water and Its Application to van der Waals Forces.

Electrical voltage externally applied between parallel-plate electrodes can cause colloidal particles located near one of the electrodes to aggregate; thus monodisperse particles can be driven to form hexagonally close-packed arrays which can be useful in the fabrication of photonic materials. The mechanism for lateral motion of particles on the electrode surface is electroosmotic flow driven by the action of the applied electric field acting either on the equilibrium charges in the diffuse layer of the particles (ECEO) or on charge induced by the electric field on the surface of the electrode (ICEO). For steady currents, ECEO dominates whereas ICEO dominates for high-frequency alternating current. For intermediate frequencies (10 Hz to 1 kHz) both mechanisms are active. This critical review attempts to integrate concepts from electrochemistry and colloid chemistry to understand this electrokinetic phenomenon.

2-D assembly of colloidal particles on a planar electrode

We are presenting the first measurements of purely steric levitation obtained with total internal reflection microscopy (TIRM), using F108 Pluronic triblock adsorbed to a polystyrene particle and a polystyrene film. The steric repulsion measured in these studies appears as a virtual hard wall. We report measurements which simultaneously detect both the “steric” and “hydrodynamic” edges of adsorbed polymer layers. The steric edge occurs at a few nanometers larger separations than the hydrodynamic edge. These measurements are also the first indication of the effects of adsorbed polymer on the net van der Waals attraction in good solvent conditions. The adsorbed polymer strengthens the net attraction for the same separation between the PS substrates but weakens the attraction for the same separation between the outer edges. It is also shown that attraction at small separations is dominated by the properties of the polymer layer, and attraction at large separations is dominated by the properties of the substr...

Dennis C. Prieve

Papers

The surface potential of and double-layer interaction force between surfaces characterized by multiple ionizable groups.

Brownian motion of a hydrosol particle in a colloidal force field

The Dielectric Function for Water and Its Application to van der Waals Forces.

2-D assembly of colloidal particles on a planar electrode

Forces and Hydrodynamic Interactions between Polystyrene Surfaces with Adsorbed PEO−PPO−PEO