arXiv subjects

Darrell Velegol

Publications and source records attributed to Darrell Velegol.

8 recordsLinked to original sources

arXiv2026

Colloidal Systems

This book has one primary goal: To get you moving quickly from learning basic principles of colloid science, to designing colloidal systems for function or motion in the lab. It is not intended to provide encyclopedic knowledge about all aspects of colloid and surface science; rather, I have chosen not to include many topics of genuine importance. In reaching the intended goal, I use two parallel strategies. First, I give practical results that can be used immediately. If there is a cumbersome calculation, I have worked to reframe it into a table or figure. This is the case for Hamaker constants and hydrodynamics of spheroids, for instance. Second, I describe the physics of various colloidal phenomena, so that you as a researcher can think about alternative strategies. In balancing algorithms with explanations, I provide plug-and-chug example problems to familiarize you with units, constants, and typical values. The practice problems provide extensions to the most basic theory that open new possibilities, while also giving results that are useful in practical and research studies. Throughout the book, I provide data we commonly use for viscosities, zeta potentials, ionic phenomena, and other parameters. Rather than providing every reference and every technique, I have provided those references and techniques that we most often use in our lab. In the end, this book aims not to be an exhaustive study of colloids, but rather to be a doorway to producing desired colloidal systems as quickly as possible.

cond-mat.soft↗

arXiv2026

Nanobubbles, pristine emulsions, high ionic strength electrokinetics -- paradoxes of Colloid and Interface Science

There are three paradoxes in the modern Colloid and Interface Science supported by large bulk of experimental evidence and contradicting classical theoretical models: - Electrokinetics at high ionic strength; - Nanobubbles having life span on scale of days and weeks without any surface stabilization; - Pristine emulsions having life span on scale of days and weeks without any surface stabilization. We overview many dozens of experimental papers by broad spectrum of scientific groups from many countries. We consider this vast experimental data as unambiguous evidence that these phenomena exist. On other hand, classical theoretical models deny such possibility. This contradiction between experiment and theory justifies introduction of new theoretical models. We overview these models. It turns out that there is one common feature between most promising of them -- assumption regarding structured water layer at hydrophobic interface. That is why we combine these three phenomena in this review. There is extensive literature on century old idea of the structured water layer, experimental and theoretical. We overview this literature, which provides convincing support to this hypothesis. We discuss existing theoretical models that incorporate the structured interfacial water layer for the successful explanation of these phenomena in more detail. Theoretical model of electrokinetics at high ionic strength was developed several decades ago. Theoretical model explaining paradoxical longevity of nanobubbles and pristine emulsions by interaction between structured water layer and electric double layer is more recent.

cond-mat.soft↗

arXiv2025

Evolution of surfactant-free 'pristine' emulsions

The term pristine interface was introduced by Beattie and Djerdjev 20 years ago for emulsions that consist of only water and oil with no surfactant. They are different from Pickering emulsions, which are also surfactant-free but stabilized with colloidal particles. In contrast to previous studies, we monitor the kinetics of the initial stages of emulsion formation. We conducted such tests in an open setup when samples are open to air and CO2 content in the water varies, and in closed setup when samples are isolated with fixed CO2 content. For the open setup, sonication and initial pH > 9 leads to emulsions with high zeta potential and sub-micron droplet size. There are two evolution patterns: short- and long-terms. The short term lasts about 1 day and has changing pH and zeta potential, but almost constant droplet size. The long term is is over several days or even weeks, with droplet size increase toward saturation value (rate dependent on mixing conditions), with pH and zeta potential remaining constant. Emulsification at the closed setup is much less pronounced and pH remains constant. This difference points to the importance of adsorbed CO2 and related carbonate ions in the formation of pristine emulsions and charging droplets interfaces. We hypothesize the existence of structured water molecule layer at the interface, following Eastoe and Ellis. The Electric Double Layer exerts a (dielectrostatic) force on the water dipole moments in this layer that compensates the Kelvins pressure. The droplet size from this model is close to our measurements. Also, there is a repulsion of the water dipole moments, which compensates for the surface tension parallel to the interface. After ruling out alternative hypotheses with our data, we conclude that the model suggested for explaining the stability of nano-bubbles is also consistent with our results for these pristine emulsions.

physics.chem-ph↗

arXiv2020

Entanglement and weak interaction driven mobility of small molecules in polymer networks

Diffusive transport of small molecules within the internal structures of biological and synthetic material systems is complex because the crowded environment presents chemical and physical barriers to mobility. We explored this mobility using a synthetic experimental system of small dye molecules diffusing within a polymer network at short time scales. We find that the diffusion of inert molecules is inhibited by the presence of the polymers. Counter-intuitively, small, hydrophobic molecules display smaller reduction in mobility and also able to diffuse faster through the system by leveraging crowding specific parameters. We explained this phenomenon by developing a de novo model and using these results, we hypothesized that non-specific hydrophobic interactions between the molecules and polymer chains could localize the molecules into compartments of overlapped and entangled chains where they experience microviscosity, rather than macroviscosity. We introduced a characteristic interaction time parameter to quantitatively explain experimental results in the light of frictional effects and molecular interactions. Our model is in good agreement with the experimental results and allowed us to classify molecules into two different mobility categories solely based on interaction. By changing the surface group, polymer molecular weight, and by adding salt to the medium, we could further modulate the mobility and mean square displacements of interacting molecules. Our work has implications in understanding intracellular diffusive transport in microtubule networks and other systems with macromolecular crowding and could lead to transport enhancement in synthetic polymer systems.

cond-mat.soft↗

arXiv2018

A Theory of Enzyme Chemotaxis: Comparison Between Experiment and Model

Enzymes show two distinct transport behaviors in the presence of their substrates in solution. First, their diffusivity enhances with increasing substrate concentration. In addition, enzymes perform directional motion toward regions with high substrate concentration, termed chemotaxis. While a variety of enzymes has been shown to undergo chemotaxis, there remains a lack of quantitative understanding of the phenomenon. Here, we provide a general expression for the active movement of an enzyme in a concentration gradient of its substrate. The proposed model takes into account both the substrate-binding and catalytic turnover step, as well as the enhanced diffusion effect. We have experimentally measured the chemotaxis of a fast and a slow enzyme: urease under catalytic conditions, and hexokinase for both full catalysis and for simple non-catalytic substrate binding. There is good agreement between the proposed model and the experiments. The model is general, has no adjustable parameters, and only requires three experimentally defined constants to quantify chemotaxis: enzyme-substrate binding affinity (Kd), Michaelis-Menten constant (KM) and level of diffusion enhancement in the associated substrate (α).

physics.bio-ph↗

arXiv2006

Fully retarded van der Waals interaction between dielectric nanoclusters

The fully retarded dispersion interaction between an atom and a cluster or between two clusters is calculated. Results obtained with two different methods are compared. One is to consider a cluster as a collection of many atoms and evaluate the sum of two-body and three-body interatomic interactions, a common assumption. The other method, valid at large separation, is to consider each cluster as a point particle, characterized by a polarizability tensor, and evaluate the inter-cluster interaction. This method employs the static polarizability, evaluated by including all many-body (MB) intra-cluster atomic interactions self-consistently, which yields the full inter-cluster interaction, including all MB terms. A comparison of the results from the two methods reveals that the contribution of the higher-than-three-body MB interactions is always attractive and non-negligible, with a relative importance that varies with geometry. The procedure is quite general and is applicable to any shape or size of dielectric clusters, in principle. We present numerical results for clusters composed of atoms with polarizability consistent with silica, for which the higher-than-three-body MB correction term can be as high as 42% of the atomic pair-wise sum. The full result is quite sensitive to the anisotropic structure of the cluster, in contrast to the result found in the additive case, which is orientation independent. We also present a power law expansion of the total van der Waals (VDW) interaction as a series of n-body interaction terms.

physics.atm-clus↗

arXiv2005

Static Polarizabilities of Dielectric Nanoclusters

A cluster consisting of many atoms or molecules may be considered, in some circustances, to be a single large molecule with a well defined polarizability. Once the polarizability of such a cluster is known, one can evaluate certain properties, e.g. the cluster's van der Waals interactions, using expressions derived for molecules. In the present work, we evaluate the static polarizability of a cluster using a microscopic method that is exact within the linear and dipolar approximations. Numerical examples are presented for various shapes and sizes of clusters composed of identical atoms, where the term "atom" actually refers to a generic constituent, which could be any polarizable entity. The results for the cluster's polarizabilities are compared with those obtained by assuming simple additivity of the constituents' atomic polarizabilities; in many cases, the difference is large, demonstrating the inadequacy of the additivity approximation. Comparison is made (for symmetrical geometries) with results obtained from continuum models of the polarizability. Also, the surface effects due to the nonuniform local field near a surface or edge are shown to be significant.

cond-mat.mtrl-sci↗

arXiv2003

Three-body interactions involving clusters and films

The three body (triple dipole) interaction of Axilrod, Teller and Muto (ATM) contributes 5 to 10 % of the total energy of condensed phases of inert elements. It is shown in this paper for clusters and films that a much larger or smaller ATM contribution can arise for other geometries or other atomic species. The ratio R of the three body interaction energy to the two body energy is evaluated for a wide variety of configurations. This ratio varies considerably with the geometry. For highly polarizable atoms in certain geometries, the magnitude of the three body energy is comparable to that of the two body energy and can be either attractive or repulsive. Systematic trends are established and explained.

cond-mat.soft↗