arXiv subjects

Sela Samin

Publications and source records attributed to Sela Samin.

At least 19 recordsLinked to original sources

arXiv2020

Universal motion of mirror-symmetric microparticles in confined Stokes flow

Comprehensive understanding of particle motion in microfluidic devices is essential to unlock novel technologies for shape-based separation and sorting of microparticles like microplastics, cells and crystal polymorphs. Such particles interact hydrodynamically with confining surfaces, thus altering their trajectories. These hydrodynamic interactions are shape-dependent and can be tuned to guide a particle along a specific path. We produce strongly confined particles with various shapes in a shallow microfluidic channel via stop flow lithography. Regardless of their exact shape, particles with a single mirror plane have identical modes of motion: in-plane rotation and cross-stream translation along a bell-shaped path. Each mode has a characteristic time, determined by particle geometry. Furthermore, each particle trajectory can be scaled by its respective characteristic times onto two master curves. We propose minimalistic relations linking these timescales to particle shape. Together these master curves yield a trajectory universal to particles with a single mirror plane.

physics.flu-dyn↗

arXiv2019

Self-Thermoelectrophoresis at Low Salinity

A locally heated Janus colloid can achieve motion in a fluid through the coupling of dissolved ions and the medium's polarizibility to an imposed temperature gradient, an effect known as self-thermo(di)electrophoresis. We numerically study the self-propulsion of such a "hot swimmer" in a monovalent electrolyte solution using the finite-element method. The effect of electrostatic screening for intermediate and large Debye lengths is charted and we report on the fluid flow generated by self-thermoelectrophoresis. We obtain excellent agreement between our analytic theory and numerical calculations in the limit of high salinity, validating our approach. At low salt concentrations, we consider two analytic approaches and use Teubner's integral formalism to arrive at expressions for the speed. These expressions agree semi-quantitatively with our numerical results for conducting swimmers, providing further validation. Our numerical approach provides a solid framework against the strengths and weaknesses of analytic theory can be appreciated and which should benefit the realization and analysis of further experiments on hot swimming.

cond-mat.soft↗

arXiv2017

Tuning the motility and directionality of self-propelled colloids

Microorganisms are able to overcome the thermal randomness of their surroundings by harvesting energy to navigate in viscous fluid environments. In a similar manner, synthetic colloidal microswimmers are capable of mimicking complex biolocomotion by means of simple self-propulsion mechanisms. Although experimentally the speed of active particles can be controlled by e.g. self-generated chemical and thermal gradients, an in-situ change of swimming direction remains a challenge. In this work, we study self-propulsion of half-coated spherical colloids in critical binary mixtures and show that the coupling of local body forces, induced by laser illumination, and the wetting properties of the colloid, can be used to finely tune both the colloid's swimming speed and its directionality. We experimentally and numerically demonstrate that the direction of motion can be reversibly switched by means of the size and shape of the droplet(s) nucleated around the colloid, depending on the particle radius and the fluid's ambient temperature. Moreover, the aforementioned features enable the possibility to realize both negative and positive phototaxis in light intensity gradients. Our results can be extended to other types of half-coated microswimmers, provided that both of their hemispheres are selectively made active but with distinct physical properties.

cond-mat.soft↗

arXiv2017

Calculating the motion of highly confined, arbitrary-shaped particles in Hele-Shaw channels

We combine theory, numerical calculations, and experiments to accurately predict the motion of anisotropic particles in shallow microfluidic channels, in which the particles are strongly confined in the vertical direction. We formulate an effective quasi-two-dimensional description of the Stokes flow around the particle via the Brinkman equation, which can be solved in a time that is two orders of magnitude faster than the three-dimensional problem. The computational speedup enables us to calculate the full trajectories of particles in the channel. To test our scheme, we study the motion of dumbbell-shaped particles that are produced in a microfluidic channel using `continuous flow lithography'. Contrary to what was reported in earlier work (Uspal et al., Nature communications 4 (2013)), we find that the reorientation time of a dumbbell particle in an external flow exhibits a minimum as a function of its disk size ratio. This finding is in excellent agreement with new experiments, thus confirming the predictive power of our scheme.

cond-mat.soft↗

arXiv2017

Microphase separation in oil-water mixtures containing hydrophilic and hydrophobic ions

We develop a lattice-based Monte Carlo simulation method for charged mixtures capable of treating dielectric heterogeneities. Using this method, we study oil-water mixtures containing an antagonistic salt, with hydrophilic cations and hydrophobic anions. Our simulations reveal several phases with a spatially modulated solvent composition, in which the ions partition between water-rich and water-poor regions according to their affinity. In addition to the recently observed lamellar phase, we find tubular, droplet, and even gyroid phases reminiscent of those found in block copolymers and surfactant systems. Interestingly, these structures stem from ion-mediated interactions, which allows for tuning of the phase behavior via the concentrations, the ionic properties, and the temperature.

cond-mat.soft↗

arXiv2017

Colloid-oil-water-interface interactions in the presence of multiple salts: charge regulation and dynamics

We theoretically and experimentally investigate colloid-oil-water-interface interactions of charged, sterically stabilized, poly(methyl-methacrylate) colloidal particles dispersed in a low-polar oil (dielectric constant $ε=5-10$) that is in contact with an adjacent water phase. In this model system, the colloidal particles cannot penetrate the oil-water interface due to repulsive van der Waals forces with the interface whereas the multiple salts that are dissolved in the oil are free to partition into the water phase. The sign and magnitude of the Donnan potential and/or the particle charge is affected by these salt concentrations such that the effective interaction potential can be highly tuned. Both the equilibrium effective colloid-interface interactions and the ion dynamics are explored within a Poisson-Nernst-Planck theory, and compared to experimental observations.

cond-mat.soft↗

arXiv2017

Solvo-osmotic flow in electrolytic mixtures

We show that an electric field parallel to an electrically neutral surface can generate flow of electrolytic mixtures in small channels. We term this solvo-osmotic flow, since the flow is induced by the asymmetric preferential solvation of ions at the liquid-solid interface. The generated flow is comparable in magnitude to the ubiquitous electro-osmotic flow at charged surfaces, but for a fixed surface charge density, it differs qualitatively in its dependence on ionic strength. Solvo-osmotic flow can also be sensitively controlled with temperature. We derive a modified Helmholtz-Smoluchowski equation that accounts for these effects.

physics.flu-dyn↗

arXiv2016

Interplay between adsorption and hydrodynamics in nanochannels: towards tunable membranes

We study how adsorption of a near-critical binary mixture in a nanopore is modified by flow inside the pore. We identify three types of steady states upon variation of the pore Péclet number (${\rm Pe}_p$), which can be reversibly accessed by the application of an external pressure. Interestingly, for small ${\rm Pe}_p$ the pore acts as a weakly selective membrane which separates the mixture. For intermediate ${\rm Pe}_p$, the flow effectively shifts the adsorption in the pore thereby opening possibilities for enhanced and tunable solute transport through the pore. For large ${\rm Pe}_p$, the adsorption is progressively reduced inside the pore, accompanied by a long ranged dispersion of the mixture far from the pore.

cond-mat.soft↗

arXiv2016

Tuning colloid-interface interactions by salt partitioning

We show that the interaction of an oil-dispersed colloidal particle with an oil-water interface is highly tunable from attractive to repulsive, either by varying the sign of the colloidal charge via charge regulation, or by varying the difference in hydrophilicity between the dissolved cations and anions. In addition, we investigate the yet unexplored interplay between the self-regulated colloidal surface charge distribution with the planar double layer across the oil-water interface and the spherical one around the colloid. Our findings explain recent experiments and have direct relevance for tunable Pickering emulsions.

cond-mat.soft↗

arXiv2016

Reversible Pore Gating in Aqueous Mixtures via External Potential

We examine theoretically porous membranes in aqueous mixtures. We show that large membrane pores can be reversibly gated from `off' (co-solvent-rich, poor conductor of ions and other solutes) to 'on' (water-rich, good conductor) states by an external potential. The transition voltage or charge for switching depends on the membrane hydrophilicity/hydrophobicity, the salt content, the preferential solvation of the salt ions, and the temperature. These parameters also determine whether the filling transition is abrupt or gradual.

cond-mat.soft↗

arXiv2016

A Landau-de Gennes theory for hard colloidal rods: defects and tactoids

We construct a phenomenological Landau-de Gennes theory for hard colloidal rods by performing an order parameter expansion of the chemical-potential dependent grand potential. By fitting the coefficients to known results of Onsager theory, we are not only able to describe the isotropic-nematic phase transition as function of density, including the well-known density jump, but also the isotropic-nematic planar interface. The resulting theory is applied in calculations of the isotropic core size in a radial hedgehog defect, the density dependence of linear defects of hard rods in square confinement, and the formation of a nematic droplet in an isotropic background.

cond-mat.soft↗

arXiv2016

Dense ionic fluids confined in planar capacitors: in- and out-of-plane structure from classical density functional theory

The ongoing scientific interest in the properties and structure of electric double layers (EDLs) stems from their pivotal role in (super)capacitive energy storage, energy harvesting, and water treatment technologies. Classical density functional theory (DFT) is a promising framework for the study of the in- and out-of-plane structural properties of double layers. Supported by molecular dynamics simulations, we demonstrate the adequate performance of DFT for analyzing charge layering in the EDL perpendicular to the electrodes. We discuss charge storage and capacitance of the EDL and the impact of screening due to dielectric solvents. We further calculate, for the first time, the in-plane structure of the EDL within the framework of DFT. While our out-of-plane results already hint at structural in-plane transitions inside the EDL, which have been observed recently in simulations and experiments, our DFT approach performs poorly in predicting in-plane structure in comparison to simulations. However, our findings isolate fundamental issues in the theoretical description of the EDL within the primitive model and point towards limitations in the performance of DFT in describing the out-of-plane structure of the EDL at high concentrations and potentials.

cond-mat.soft↗

arXiv2015

Self-propulsion mechanism of active Janus particles in near-critical binary mixtures

Gold-capped Janus particles immersed in a near-critical binary mixture can be propelled using illumination. We employ a non-isothermal diffuse interface approach to investigate the self-propulsion mechanism of a single colloid. We attribute the motion to body forces at the edges of a micronsized droplet that nucleates around the particle. Thus, the often-used concept of a surface velocity cannot account for the self-propulsion. The particle's swimming velocity is related to the droplet shape and size, which is determined by a so-called critical isotherm. Two distinct swimming regimes exist, depending on whether the droplet partially or completely covers the particle. Interestingly, the dependence of the swimming velocity on temperature is non-monotonic in both regimes.

cond-mat.soft↗

arXiv2015

Fundamental measure theory for the electric double layer: implications for blue-energy harvesting and water desalination

Capacitive mixing (CAPMIX) and capacitive deionization (CDI) are promising candidates for harvesting clean, renewable energy and for the energy efficient production of potable water, respectively. Both CAPMIX and CDI involve water-immersed porous carbon (supercapacitors) electrodes at voltages of the order of hundreds of millivolts, such that counter-ionic packing is important for the electric double layer (EDL) which forms near the surface of these porous materials. Thus, we propose a density functional theory (DFT) to model the EDL, where the White-Bear mark II fundamental measure theory functional is combined with a mean-field Coulombic and a mean spherical approximation-type correction to describe the interplay between dense packing and electrostatics, in good agreement with molecular dynamics simulations. We discuss the concentration-dependent potential rise due to changes in the chemical potential in capacitors in the context of an over-ideal theoretical description and its impact on energy harvesting and water desalination. Compared to less elaborate mean-field models our DFT calculations reveal a higher work output for blue-energy cycles and a higher energy demand for desalination cycles.

cond-mat.soft↗

arXiv2014

Experimental Demonstration of the Stabilization of Colloids by Addition of Salt

We demonstrate a general non--Derjaguin-Landau-Verwey-Overbeek method to stabilize colloids in liquids. By this method, colloidal particles that initially form unstable suspension and sediment from the liquid are stabilized by the addition of salt to the suspending liquid. Yet, the salt is not expected to adsorb or directly interact with the surface of the colloids. For the method to work, the liquid should be a mixture, and the salt needs to be antagonistic such that each ion is preferentially solvated by a different component of the mixture. The stabilization may depend on the salt content, mixture composition, or distance from the mixture's coexistence line.

cond-mat.soft↗

arXiv2013

Stabilization of charged and neutral colloids in salty mixtures

We present a mechanism for the stabilization of colloids in liquid mixtures without use of surfactants or polymers. When a suitable salt is added to a solvent mixture, the coupling of the colloid's surface chemistry and the preferential solvation of ions leads to a repulsive force between colloids that can overcome van der Waals attraction. This repulsive force is substantial in a large range of temperatures, mixture composition and salt concentrations. The increased repulsion due to addition of salt occurs even for charged colloids. This mechanism may be useful in experimental situations where steric stabilization with surfactants or polymers is undesired.

cond-mat.soft↗

arXiv2013

Vapor-liquid coexistence of the Stockmayer fluid in nonuniform external fields

We investigate the structure and phase behavior of the Stockmayer fluid in the presence of nonuniform electric fields using molecular simulation. We find that an initially homogeneous vapor phase undergoes a local phase separation in a nonuniform field due to the combined effect of the field gradient and the fluid vapor--liquid equilibrium. This results in a high density fluid condensing in the strong field region. The system polarization exhibits a strong field dependence due to the fluid condensation.

cond-mat.soft↗

arXiv2012

Controlling the fluid-fluid mixing-demixing phase transition with electric fields

We review recent theoretical advances on controlling the fluid-fluid phase transition with electric fields. Using a mean-field approach, we compare the effects of uniform versus non-uniform electric fields, and show how non-uniform fields are better at altering the phase diagram. Focusing on non-uniform fields, we then discuss the behavior of the fluid concentration profile and the parameters (temperature, fluid concentration, etc.) that control the location of the fluid-fluid interface from both equilibrium and dynamic perspectives.

cond-mat.soft↗