Universal Dripping and Jetting in a Transverse Shear Flow

One particularly efficient approach to making emulsions having monosized droplets is to push a fluid through an orifice into a transverse flow of a second immiscible fluid. We find that, at an intermediate particle Reynolds number, the final droplet size can be readily computed using a simple force balance. Remarkably like the well-known dripping faucet, this system displays both dripping and jetting behavior, controlled by the capillary, Weber and Ohnesorge numbers of the relevant fluids, and interesting nonlinear behavior such as period doubling near the transition between these two regimes

Line Optical Tweezers Instrument for Measuring Nanoscale Interactions and Kinetics

We describe an optical tweezers instrument for measuring short-ranged colloidal interactions, based on a combination of a continuous wave line optical tweezers, high speed video microscopy, and laser illumination. Our implementation can measure the separation of two nearly contacting microspheres to better than 4 nm at rates in excess of 10 kHz. A simple image analysis algorithm allows us to sensibly remove effects from diffraction blurring and microsphere image overlap for separations ranging from contact to at least 100 nm. The result is a versatile instrument for measuring steric, chemical and single-molecular interactions and dynamics, with a force resolution significantly better than achievable with current atomic force microscopy. We demonstrate the effectiveness of the instrument with measurements of the pair interactions and dynamics of microspheres in the presence of transient molecular bridges of DNA or surfactant micelles

Computational Analysis of Binary Segregation During Colloidal Crytallization with DNA-mediated Interactions

A detailed computational study of compositional segregation during growth of colloidal binary solid-solution crystals is presented. Using a comprehensive set of Metropolis Monte Carlo simulations, we probe the influence of colloid size, interaction strength, and interaction range on the segregation process. The results are interpreted in terms of a simple, but descriptive mechanistic model that allows us to connect to studies of binary segregation in atomic systems. The validity of Metropolis Monte Carlo simulations for the nonequilibrium phenomena investigated in this work is established theoretically and by connections to Brownian dynamics and molecular dynamics simulations. It is demonstrated that standard Metropolis Monte Carlo, properly applied, can provide an efficient framework for studying many aspects of crystallization in colloidal systems

Fragility and Mechanosensing in a Thermalized Cytoskeleton Model with Forced Protein Unfolding

We describe a model of cytoskeletal mechanics based on the force-induced conformational change of protein cross-links in a stressed polymer network. Slow deformation of simulated networks containing cross-links that undergo repeated, serial domain unfolding leads to an unusual state — with many cross-links accumulating near the critical force for further unfolding. This state is robust to thermalization and does not occur in similar protein unbinding based simulations. Moreover, we note that the unusual configuration of near-critical protein cross-links in the fragile state provides a physical mechanism for the chemical transduction of cell-level mechanical strain and extra-cellular matrix stiffness

Exploring canyons in glassy energy landscapes using metadynamics

The complex physics of glass forming systems is controlled by the structure of the low energy portions of their potential energy landscapes. Here, we report that a modified metadynamics algorithm efficiently explores and samples low energy regions of such high-dimensional landscapes. In the energy landscape for a model foam, metadynamics finds and descends meandering `canyons' in the landscape, which contain dense clusters of energy minima along their floors. Similar canyon structures in the energy landscapes of two model glass formers--hard sphere fluids and the Kob-Andersen glass--allow metadynamics to reach low energies. In the hard sphere system, fluid configurations are found to form continuous regions that cover the canyon floors, but only up to a volume fraction close to that predicted for kinetic arrest. For the Kob-Andersen glass former, metadynamics reaches the canyons' ends with modest computational effort; with the lowest energies found approaching the predicted Kauzmann limit.Comment: 7 pages and 5 figures, plus appendice

Colloidal Interactions and Self-Assembly Using DNA Hybridization

The specific binding of complementary DNA strands has been suggested as an ideal method for directing the controlled self-assembly of microscopic objects. We report the first direct measurements of such DNA-induced interactions between colloidal microspheres, as well as the first colloidal crystals assembled using them. The interactions measured with our optical tweezer method can be modeled in detail by well-known statistical physics and chemistry, boding well for their application to directed selfassembly. The microspheres’ binding dynamics, however, have a surprising power-law scaling that can significantly slow annealing and crystallization

Role of configurational entropy in the thermodynamics of clusters of point defects in crystalline solids

The internal configurational entropy of point defect clusters in crystalline silicon is studied in detail by analyzing their potential energy landscapes. Both on-lattice and off-lattice calculation approaches are employed to demonstrate the importance of off-lattice configurational states that arise due to a large number of inherent structures (local minima) in the energy landscape generated by the interatomic potential function. The resulting cluster configurational entropy of formation is shown to exhibit behavior that is qualitatively similar to that observed in supercooled liquids and amorphous solids and substantially alters the thermodynamic properties of point defect clusters in crystals at high temperature. This behavior is shown to be independent of interatomic potential and cluster type, and suggests that defects in crystals at high temperature should be generally described by a quasicontinuous collection of nondegenerate states rather than as a single ground state structure. The modified thermodynamic properties of vacancy clusters at high temperature are found to explain a longstanding discrepancy between simulation predictions and experimental measurements of vacancy aggregation dynamics in silicon