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

A single particle (365–640 μm) is injected into a 0.1% w/w solution of polyisobutylene (Mv=1.5×106), dissolved in polybutene (Mn=800), held between a plate and a 21‐cm disc. When rotation of the disc is begun at 4–9 rpm, with a disc‐plate separation of 3.6–6.6 mm, an anomalous discontinuity in the direction of lateral migration is observed at an initial radial location r=rcr: for r rcr, the particle migrates radially outward and vertically toward a plane midway between the disc and plate. By improving the apparatus used in Part I [J. Rheol. 28, 381 (1984)], more reproducible measurements of rcr are obtained. rcr is found to be inversely proportional to rotation speed, directly proportional to disc‐plate separation, and independent of particle radius. The proportionality constant, 6.3/s, represents a critical shear rate for the direction of lateral migration in this fluid and this geometry. A qualitative interpretation for t...

Anomalous Migration of a Rigid Sphere in Torsional Flow of a Viscoelastic Fluid. II: Effect of Shear Rate

When light strikes a body, it exerts a force which arises from the momentum of the photons. Here we use ray optics to predict the force on a sphere (much larger than the wavelength of light) exerted by a laser beam of low divergence (in contrast with high divergence beams used to form optical traps) to determine if the magnitude of force attainable is large enough to dislodge microscopic particles adhered to surfaces. For simple dielectric microspheres (e.g. polystyrene or glass) the maximum axial force of about 0.6 nN/W is exerted by an ideal uniform beam, concentric with the sphere, when the beam radius is comparable to the sphere radius. Donut and Gaussian beams produce slightly smaller maximum forces. Focusing the beam to a waist size much smaller than the sphere and aiming it at the edge of the sphere can produce axial forces of 5 nN/W, independent of beam shape. Unfortunately, this eccentricity also generates a torque which would stress the adhesive nonuniformly. The absolute maximum force of 8.9 nN/W (2/υ, where υ is the speed of light through the surrounding medium) can be obtained without torque using a concentric beam if the sphere is constructed of a highly reflective material. Our results suggest that radiation forces can be used to detach microscopic particles adhered to surfaces.

Force exerted by a laser beam on a microscopic sphere in water : designing for maximum axial force

Electric fields are commonly used to deposit colloidal particles on electrode surfaces and can even be used in directed assembly. The electric field beneath each particle changes as the particle approaches the wall; the proximity of the wall breaks the fore/aft symmetry and drives complicated flows that exert forces on the particle. While two limiting cases have been partially analyzed, constant electrode potential and uniform current density, the full problem has not been explored. Here, the electroosmotic flows in the region between the particle and the electrode are analyzed and the forces are computed for arbitrary electrode kinetic boundary conditions. Finite element analysis is employed to explore the effect of the current distribution beneath a particle on the net force acting on it. Previously established dimensionless kinetic parameters are used to scale between the two limiting cases. The forces on particles are an order of magnitude larger than the bulk electrophoretic force and are profoundly sensitive to the current distribution beneath the particle as it approaches the electrode.

The effect of electrode kinetics on electrophoretic forces

According to the Rayleigh−Debye (RG) theory of light scattering, the intensity of forward scattering is proportional to the volume-squared of the scatterers, independent of their shape or orientation This makes small-angle light scattering (SALS) attractive as a tool for studying the kinetics of flocculation of model latexes, where the conformation of elemental particles in any floc is unknown In preparation for such a study using a modified version of the apparatus of Lips and Willis (1973), we experimentally determine the limits under which SALS produced by a He−Ne laser can be used for sizing of monodisperse polystyrene latexes For every particle in the sample volume to experience the same intensity of incident light (ie for negligible extinction), the particle concentration must be less than cmax, where cmaxld6 = (108 ± 009) × 10-26 m4, l is the pathlength and d is the particle diameter For c < cmax, the scattering per particle at 2° is proportional to d6 provided d is less than 1 μm, which is

Small-Angle Rayleigh Scattering by Relatively Large Latex Particles

Total internal reflection microscopy (TIRM) is an optical technique for monitoring Brownian fluctuations in the separation between a single microscopic sphere and a flat plate in aqueous media. The sphere is levitated above the plate by colloidal forces such as double-layer or steric repulsion. Changes in elevation as small as 1 nm can be detected by measuring the light scattered by a single sphere when illuminated by an evanescent wave. From the Boltzmann distribution of elevations sampled by the sphere over a long time, the potential energy (PE) profile can be determined with a resolution of about 0.1kT. By corrupting clean data (having a mean scattering intensity of Īs and a standard deviation of σs) obtained by Brownian dynamics simulations with various levels of additive background noise (having a mean background intensity of Īb and a standard deviation of σb), we show how the PE profiles obtained from TIRM are distorted. Increasing Īb narrows the profile and shifts it toward smaller elevations; conv...

Dennis C. Prieve

Papers

Anomalous Migration of a Rigid Sphere in Torsional Flow of a Viscoelastic Fluid. II: Effect of Shear Rate

Force exerted by a laser beam on a microscopic sphere in water : designing for maximum axial force

The effect of electrode kinetics on electrophoretic forces

Small-Angle Rayleigh Scattering by Relatively Large Latex Particles

Total Internal Reflection Microscopy: Distortion Caused by Additive Noise