This book covers the physical side of colloidal science from the individual forces acting between particles smaller than a micrometer that are suspended in a liquid, through the resulting equilibrium and dynamic properties. A variety of internal forces both attractive and repulsive act in conjunction with Brownian motion and the balance between them all decides the phase behaviour. On top of this various external fields, such as gravity or electromagnetic fields, diffusion and non-Newtonian rheology produce complex effects, each of which is of important scientific and technological interest. The authors aim to impart a sound, quantitative understanding based on fundamental theory and experiments with well-characterised model systems. This broad grasp of the fundamentals lends insight and helps to develop the intuitive sense needed to isolate essential features of the technological problems and design critical experiments. The main prerequisites for understanding the book are basic fluid mechanics, statistical mechanics and electromagnetism, though self contained reviews of each subject are provided at appropriate points. Some facility with differential equations is also necessary. Exercises are included at the end of each chapter, making the work suitable as a textbook for graduate courses in chemical engineering or applied mathematics. It will also be useful as a reference for individuals in academia or industry undertaking research in colloid science.

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Thin films of latex dispersions containing particles of high glass transition temperature generally crack while drying under ambient conditions. Experiments with particles of varying radii focused on conditions for which capillary stresses normal to the film deform the particles elastically and generate tensile stresses in the plane of the film. Irrespective of the particle size, the drying film contained, simultaneously, domains consisting of a fluid dispersion, a fully dried packing of deformed spheres, and a close packed array saturated with water. Interestingly, films cast from dispersions containing 95-nm sized particles developed tensile stresses and ultimately became transparent even in the absence of water, indicating that van der Waals forces can deform the particles. Employing the stress-strain relation for a drying latex film along with the well-known Griffith's energy balance concept, we calculate the critical stress at cracking and the accompanying crack spacing, in general agreement with the observed values.

The deformation of particles, to produce a structure without voids, has been an issue of contention in the film formation community for many years. Four different mechanisms have been proposed. Three involve homogeneous deformation throughout the film, although all are built on the deformation of two isolated particles, described in the viscous limit by Frenkel and in the elastic limit by Hertz and Johnson, Kendall, and Roberts. We derive a linear viscoelastic generalization of Frenkel's model that predicts the deformation of two spheres compressed by a force, F, and surface tension, γ. The resulting equation is then embedded in field equations governing the collapse of macroscopic films. Assuming a uniaxial compression allows derivation of limits for the proposed modes of homogeneous deformation. These limits are shown as surfaces in parameter space. Since temperature alters most profoundly the rheological response of viscoelastic polymers, the controlling deformation mechanism is defined as a function of temperature. Wet sintering requires slow evaporation or a low modulus polymer and is seen at high temperatures. Capillary deformation requires the strain in the film to follow evaporation and appears at intermediate temperatures.Dry or moist sintering is then seen at the lowest temperatures, when the modulus is high and deformation is slow compared to evaporation.

We present a technique for the directed assembly and self-assembly of micrometer-scale structures based on the control of specific DNA linkages between colloidal particles. The use of DNA links combined with polymer brushes provides an effective way to regulate the range and magnitude of addressable forces between pairs (and further combinations) of different particles. We demonstrate that the autoassembly of alternate microbeads as well as their directed assembly, by using laser tweezers, is reversible. The key to reversibility is preventing the particles from falling into their van der Waals well at close distances. This goal is achieved by the use of adsorbed polymers that limit the number of DNA bridges to one to three between adjacent particles.DNA links ͉ reversible aggregation S tudies of reversible and specific adhesion between colloids are an important step toward understanding various phenomena involving molecular recognition. For instance, they are relevant in the study of cell adhesion (1), cell migration (2), or cell sorting during embryonic development (3). The knowledge and the control of the interplay between nonspecific repulsion and molecular recognition is also fundamental for biotechnological device improvements; e.g., a strategy to improve latex agglutination tests is to reduce aggregation due to nonspecific interactions (4). The present work on DNA links is also a contribution to these more general studies.Controlling and tuning interactions between particles has always been a relevant challenge both experimentally (5, 6) and theoretically (7-11). For example, Tkachenko (11) predicted diverse and unusual crystal morphologies assuming a reversible contact between particles in a binary system of colloids, in which identical particles experience repulsive interactions and differing particles experience attractive ones. Particularly, he predicted a selfassembled diamond lattice structure that would be especially relevant for photonic crystal building. His work was inspired by the work by Mirkin et al. (12), who first used DNA chains as linkers between nanoparticles to build a reversible DNA-mediated assembly of gold nanoparticles. The specificity and magnitude of the attraction is determined by the molecular recognition of complementary DNA strands and the sensitivity of hybridization to solution conditions and temperature. Experimental work involving DNA as a linker between particles has up to now mainly focused on nanosized particles (13-18). To our knowledge, only two studies (19,20) have been reported in the literature with microsized particles, but in both of them the assembly process was not reversible, with the DNA acting as a molecular bridge between the entities of a binary mixture. In this study, we focus on the reversibility of the aggregation process between microsized particles. The specificity and reversibility are proof that the interactions between the colloids are controlled by DNA and thus can be tuned. Materials and MethodsSample Preparation. DNA-functionalized polystyrene mi...

Latex films cast on a substrate open dry nonunifomly, with a drying front separating fluid domains from solidified regions passing across the film. For initial film thicknesses that are smaller than the characteristic horizontal distance, the analysis predicts surface-tension-driven horizontal flow. In a limit that ensures vertical homogeneity it is shown how a front of close-packed particles forms and propagates. Imposing a maxim u m for the capillaly pressure causes a solvent front to recede into the film. This recession is minimal, but can markedly affect the propagation of the particle front. An overall mass balance offers a solution for infinite capillaly pressure, thereby illustrating the mechanism for propagation of the front. The positions of the fronts are predicted for both infinite and finite domains as a function of the maximum capillaly pressure. Selective or nonuniform evaporation produces final film profiles, while the evaporating regions are still visible. After predictions over different size areas are made, the smallest area is compared with experiment.