# Poster session and reception

Tuesday, March 13, 2018 - 3:15pm - 4:45pm

Lind 400

**Electrohydrodynamic Effects on Colloid and Electrolyte Dynamics: Conductive-Diffusive Transport and Crystallization Kinetics. Theory and Computer Simulations.**

Claudio Contreras Aburto (Universidad Autonoma de Chiapas)

We are investigating the hydrodynamic (HI) and electrostatic (EI) interaction effects on colloidal and electrolyte transport, and on the kinetics of crystallization of charged colloids. We are specially interested on the underlyings of electrokinetic effects. Our goal is to develop theoretical and computer simulation schemes, mainly on a mesoscopic level, such as the Smoluchowski and over-damped Langevin (brownian) levels of description.**Fluid dynamics of vesicular transport in dendritic spines**

Thomas Fai (Harvard University)

We model the fluid dynamics of vesicle transport into dendritic spines, which are micron-sized structures at which neuronal postsynapses are located. Dendritic spines are characterized by their thin necks and bulbous heads, and recent high-resolution 3D images show a fascinating variety of spine morphologies. Our model, which we validate using 3D lattice Boltzmann simulations, reduces the dynamics of vesicle motion to two essential parameters representing the system geometry and elasticity and allows us to thoroughly explore phase space. Upon including competing molecular motor species that push and pull on vesicles, we observe multistability that we speculate neurons could exploit in order to control spine growth. To deal with such problems more generally, we introduce an immersed boundary method that uses a subgrid lubrication model to resolve thin fluid layers between immersed boundaries.**Electrostatic contributions in DNA phase transitions**

Prashant Purohit (University of Pennsylvania)

Our objective is to explore the implications of variation in ion concentration on the structural transitions driven by external forces in a torsionally constrained DNA molecule. A comprehensive understanding of the phase behavior of torsionally constrained DNA is useful because DNA in cells is tightly packaged and is acted upon by molecular machines in different ionic environments. We examine the mechanics of the overstretching transition, characterized by a 70% jump in contour length, wherein a mixture of B- and S-DNA converts into a mixture of S- and P-DNA through a triple point in the phase diagram. Our results are corroborated by experimental data at every step and we make predictions that are experimentally verifiable.**Curvature driven evolution of a smectic-A liquid crystal out of thermodynamic equilibrium**

Eduardo Vitral (University of Minnesota, Twin Cities)

We introduce a mesoscale model of a complex fluid to study the two phase interface separating a layered phase of uniaxial symmetry from an isotropic phase. The model is used to derive capillary and elastic contributions to local equilibrium conditions at deformed interfaces (generalized Gibbs-Thomson relations), extra stresses and their contribution to flow, and the nonequilibrium equations governing interfacial motion. Particular attention is paid to often neglected surface invariants such as the Gaussian curvature, and its role in driving changes of topology of the interface during its evolution. The methodology also lends itself to large scale computational analysis, with a parallel implemented pseudo-spectral approach. Focal conics are verified to be equilibrium shapes for the proposed phase field description. Our study is motivated by recent experiments on surface instabilities of toroidal focal conic domains in smectic films, and preliminary out of equilibrium results are shown to match some of the experimentally observed morphologies.**Non-Isothermal Electrokinetics: Energetic Variational Approach**

Pei Liu (The Pennsylvania State University)

A number of ion channels are observed to be sensitive to the temperature changes. These temperature-gated ion channels can detect the temperature thus regulate the internal homoeostasis and disease-related processes such as the thermal adaptation and the fever response. In order to understand how the temperature affects the ion channel mechanics, we develop a Poisson--Netnst--Planck--Fourier (PNPF) system through the energetic variational approach. With given form of the free energy functional and the entropy production, we achieve the mechanical equations and a temperature equation, which satisfy the laws of thermodynamics automatically. From the energy point of view, we also develop the numeric scheme which satisfy the discrete energy dissipation.**Deterministic and chaotic dynamics of rotating semiflexible particle chains**

Sibani Biswal (Rice University)

The interaction between flexible filaments and a surrounding fluid has been a topic of study for many years, due to its relevance to both naturally occurring and industrially relevant phenomena. However, the connection between a fiber’s elastic properties, the external driving forces, and the eventual dynamics of the fiber is not well understood. Fibers in the semi-flexible regime, such as microtubules and carbon nanotubes, are of particular interest, as the competition of elastic, hydrodynamic, and external forces appear to cause deviations from the scalings expected of rigid and flexible fibers. We thus investigate the dynamical behavior of semi-flexible fibers experimentally, through the use of a tunable model system consisting of linked paramagnetic particle chains; when chains are driven by an external rotating magnetic field, which exerts a torque profile analogous to that of shear flow, we observe multiple regimes of dynamical behavior. By pairing experiments, computational models, and theoretical calculations, we find that the dynamics of the system depend on the dimensionless Mason and magnetoelastic numbers, which we then use to identify and classify the regimes of different rotational modes.**Ion injection mechanism of contact angle saturation in electrowetting**

Tetsuya Yamamoto (Nagoya University)

Electrowetting is a process with which the contact angle θ(sub)w of a droplet decreases with increasing applied voltage. In a typical experiment, an aqueous droplet, immersed in an oil medium, is prepared on a dielectric film and voltage is applied between the droplet and an electrode at the opposite side of the dielectric film. With small applied voltage, cosθ(sub)w is a quadratic function of applied voltage, in agreement with the Young-Lippmann theory. For larger applied voltage, cosθ(sub)w deviates from the prediction of the Young-Lippmann theory and eventually saturates. We here use the Poisson-Boltzmann theory to predict that ions are injected from the aqueous droplet to the oil due to the large applied voltage and this deviates the contact angle from the Young-Lippmann theory. This theory predicts that the contact angle saturation is not a real saturation, but a broad maximum of cosθ(sub)w.**Electrostatic Assist of Liquid Transfer between Flat Surfaces**

Chung-Hsuan Huang (University of Minnesota, Twin Cities)

Transfer of liquid from one surface to another plays a vital role in printing processes. During liquid transfer, a liquid bridge is formed and subjected to substantial extension, but incomplete liquid transfer can produce defects that are detrimental to the operation of printed electronic devices. One strategy for minimizing these defects is to apply an electric field, a technique known as electrostatic assist (ESA). However, the physical mechanisms underlying ESA remain a mystery. To better understand these mechanisms, slender-jet models are developed for both perfect dielectric and leaky dielectric axisymmetric Newtonian liquid bridges with moving contact lines. Nonlinear partial differential equations describing the evolution of the bridge radius and interfacial charge are derived, and then solved using finite-element methods. For perfect dielectrics, application of an electric field enhances liquid transfer to the more wettable surface over a wide range of capillary numbers. The electric field modifies the pressure differences inside the liquid bridge, and as a consequence, drives liquid toward the more wettable surface. For leaky dielectrics, charge can accumulate at the liquid-air interface. Application of an electric field can augment or oppose the influence of wettability differences, depending on the direction of the electric field and the sign of the surface charge. Flow visualization experiments reveal that when an electric field is applied, more liquid is transferred to the more wettable surface due to a modified bridge shape that causes depinning of the contact line. The measured values of the amount of liquid transferred are in good agreement with predictions of the perfect dielectric model.