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

When a particle is placed in a fluid in which there is a non-uniform concentration of solute, it will move toward higher or lower concentration depending on whether the solute is attracted to or repelled from the particle surface. A quantitative understanding of this phenomenon requires that the equations representing conservation of mass and momentum within the fluid in the vicinity of the particle are solved. This is accomplished using a method of matched asymptotic expansions in a small parameter L/a, where a is the particle radius and L is the length scale characteristic of the physical interaction between solute and particle surface. This analysis yields an expression for particle velocity, valid in the limit L/a → 0, that agrees with the expression obtained by previous researchers. The result is cast into a more useful algebraic form by relating various integrals involving the solute/particle interaction energy to a measurable thermodynamic property, the Gibbs surface excess of solute Γ. An important result is that the correction for finite L/a is actually O(Γ/C∞ a), where C∞ is the bulk concentration of solute, and could be O(1) even when L/a is orders of magnitude smaller.

Motion of a particle generated by chemical gradients. Part 2. Electrolytes

https://www.andrew.cmu.edu/user/dcprieve/TIRM%20review%20article%20(preprint).pdf

Measurement of colloidal forces with TIRM

Adsorption and desorption of particles and their chromatographic separation

Total internal reflection microscopy is used to measure the total potential energy of interaction between a 6 μm polystyrene (PS) latex bead and either a bare glass microscope slide or a glass slide spin-coated with a 1 μm thick PS film, when the two interacting bodies are separated by 10−300 nm of aqueous solution having an ionic strength between 0.5 and 3 mM. In particular, these are the first measurements of van der Waals interaction between microscopic bodies of PS across water, for which the dielectric spectra are well-known. Under these conditions the bead is levitated above the slide by double-layer repulsion. After the gravitational contribution is subtracted, the potential energy profile displays a minimum of 0.5−2.3kT formed by long-range van der Waals attraction and shorter-range double-layer repulsion. The attraction was detectable at distances up to 200 nm. At separation distances greater than 100 nm (energy < 0.5kT), the measurements agree well with predictions using Lifshitz theory to predi...

Direct measurement of retarded van der waals attraction

Total internal reflection microscopy is a technique for monitoring changes in the distance between a single microscopic sphere and a flat plate by measuring the intensity of light scattered by the sphere when illuminated by an evanescent wave. A histogram of scattering intensities can be used to construct the potential energy profile as a function of distance relative to the most probable distance. Thus potential energies can be measured to within a fraction of kT while changes in distance can be measured to within 1 nm. An autocorrelation of the scattering intensities can be used to deduce an average diffusion coefficient of the sphere, which is found to be only a few percent of the Stokes–Einstein value, owing to the close proximity of the plate. The analysis of the intensity-autocorrelation function presented here can be used to deduce an absolute value for the most probable separation distance, without a priori knowledge of the functional form of the PE profile and in the presence of a constant background scattering intensity. This “hydrodynamic” separation distance is found to be within a few percent of the “optical” separation distance found independently by comparing the intensity at the most probable distance with the intensity of the same particle in contact with the plate. Since the particle does not need to be brought into contact with the plate, the hydrodynamic method is well suited for determining the absolute separation distance with deformable particles like liquid droplets, vesicles or biological cells. Moreover, the hydrodynamic separation can be immediately calculated without any additional experiments. However, accurate determination of the hydrodynamic separation requires an accurate value for the particle size, which must be determined independently.

Dennis C. Prieve

Papers

Motion of a particle generated by chemical gradients. Part 2. Electrolytes

Measurement of colloidal forces with TIRM

Adsorption and desorption of particles and their chromatographic separation

Direct measurement of retarded van der waals attraction

Hindered diffusion of colloidal particles very near to a wall: Revisited