Dynamic Partitioning of Surfactants into Nonequilibrium Emulsion Droplets.

Characterizing the propensity of molecules to distribute between fluid phases is key to describing chemical concentrations in heterogeneous mixtures and the corresponding physiochemical properties of a system. Typically, partitioning is studied under equilibrium conditions. However, some mixtures form a single phase at equilibrium but exist in multiple phases when out-of-equilibrium, such as oil-in-water emulsion droplets stabilized by surfactants. Such droplets persist for extended times but ultimately disappear due to droplet dissolution and micellar solubilization. Consequently, equilibrium properties like oil-water partition coefficients may not accurately describe out-of-equilibrium droplets. This study investigates the partitioning of nonionic surfactants between shrinking microscale oil droplets and water under nonequilibrium conditions. Quantitative mass spectrometry is used to analyze the composition of individual microdroplets over time under conditions of varying surfactant composition, concentrations, and oil molecular structures. Within minutes, nonionic surfactants partition into oil droplets, reaching a nonequilibrium steady-state concentration that can be over an order of magnitude higher than that in the aqueous phase. As the droplets solubilize over hours, the surfactants are released back into water, leading to transiently high surfactant concentrations near the droplet-water interface and the formation of a microemulsion phase with a low interfacial tension. Introducing ionic surfactants that form mixed micelles with nonionic surfactants reduces partitioning. Based on this observation, stimuli-responsive ionic surfactants are used to modulate the nonionic surfactant partitioning and trigger reversible phase separation and mixing inside binary oil droplets. This study reveals generalizable nonequilibrium states and conditions experienced by solubilizing oil droplets that influence emulsion properties.

Dynamics near planar walls for various model self-phoretic particles.

For chemically active particles suspended in a liquid solution and moving by self-phoresis, the dynamics near chemically inert, planar walls is studied theoretically by employing various choices for the activity function, i.e., the spatial distribution of the sites where various chemical reactions take place. We focus on the case of solutions composed of electrically neutral species. This analysis extends previous studies of the case that the chemical activity can be modeled effectively as the release of a "product" molecular species from parts of the surface of the particle by accounting for annihilation of the product molecules by chemical reactions, either on the rest of the surface of the particle or in the volume of the surrounding solution. We show that, for the models considered here, the emergence of "sliding" and "hovering" wall-bound states is a generic, robust feature. However, the details of these states, such as the range of parameters within which they occur, depend on the specific model for the activity function. Additionally, in certain cases there is a reversal of the direction of the motion compared to the one observed if the particle is far away from the wall. We have also studied the changes of the dynamics induced by a direct interaction between the particle and the wall by including a short-ranged repulsive component to the interaction in addition to the steric one (a procedure often employed in numerical simulations of active colloids). Upon increasing the strength of this additional component, while keeping its range fixed, significant qualitative changes occur in the phase portraits of the dynamics near the wall: for sufficiently strong short-ranged repulsion, the sliding steady states of the dynamics are transformed into hovering states. Furthermore, our studies provide evidence for an additional "oscillatory" wall-bound steady state of motion for chemically active particles due to a strong, short-ranged, and direct repulsion. This kind of particle translates along the wall at a distance from it which oscillates between a minimum and a maximum.

Dynamics of two interacting active Janus particles.

Starting from a microscopic model for a spherically symmetric active Janus particle, we study the interactions between two such active motors. The ambient fluid mediates a long range hydrodynamic interaction between two motors. This interaction has both direct and indirect hydrodynamic contributions. The direct contribution is due to the propagation of fluid flow that originated from a moving motor and affects the motion of the other motor. The indirect contribution emerges from the re-distribution of the ionic concentrations in the presence of both motors. Electric force exerted on the fluid from this ionic solution enhances the flow pattern and subsequently changes the motion of both motors. By formulating a perturbation method for very far separated motors, we derive analytic results for the translation and rotational dynamics of the motors. We show that the overall interaction at the leading order modifies the translational and rotational speeds of motors which scale as O[1/D](3) and O[1/D](4) with their separation, respectively. Our findings open up the way for studying the collective dynamics of synthetic micro-motors.

Entropic forces exerted on a rough wall by a grafted semiflexible polymer.

We study the entropic force due to a fluctuating semiflexible polymer that is grafted from one end and confined by a rigid and rough wall from the other end. We show how roughness of the wall modifies the entropic force. In addition to the perpendicular force that is present in the case of a flat wall, roughness of the wall adds a lateral component to the force. Both perpendicular and lateral components of the force are examined for different values of amplitude and wavelength of the roughness and at different temperatures. The lateral force is controlled by the local slope of the wall while the perpendicular force is only sensitive to the curvature of the wall. We show that for small compression, the entropic force is increased by increasing the curvature of the confining wall. In addition to the biophysical relevance, the results may also be useful in developing an AFM-based experimental technique for probing the roughness of surfaces.

Orbits, Spirals, and Trapped States: Dynamics of a Phoretic Janus Particle in a Radial Concentration Gradient.

A long-standing goal in colloidal active matter is to understand how gradients in fuel concentration influence the motion of phoretic Janus particles. Here, we present a theoretical description of the motion of a spherical phoretic Janus particle in the presence of a radial gradient of the chemical solute driving self-propulsion. Radial gradients are a geometry relevant to many scenarios in active matter systems and naturally arise due to the presence of a point source or sink of fuel. We derive an analytical solution for the Janus particle's velocity and quantify the influence of the radial concentration gradient on the particle's trajectory. Compared to a phoretic Janus particle in a linear gradient in fuel concentration, we uncover a much richer set of dynamic behaviors including circular orbits and trapped stationary states. We identify the ratio of the phoretic mobilities between the two domains of the Janus particle as a central quantity in tuning their dynamics. Our results provide a path for developing optimum protocols for tuning the dynamics of phoretic Janus particles and mixing fluid at the microscale. In addition, this work suggests a method for quantifying the surface properties of phoretic Janus particles, which have proven to be challenging to probe experimentally.

Memory effects in spiral diffusion of rotary self-propellers.

The coupling of deterministic rotary motion and stochastic orientational diffusion of a self-propeller leads to a spiral trajectory of the expected displacement. We extend our former analysis of spiral diffusion [Phys. Rev. E 94, 030601(R) (2016)10.1103/PhysRevE.94.030601] in the white-noise limit to a more realistic scenario of stochastic noise with Gaussian memory and orientational fluctuations driven by an Ornstein-Uhlenbeck process. A variety of dynamical regimes including crossovers from ballistic to diffusive to ballistic in the angular dynamics are determined by the inertial timescale, orientational diffusivity, and angular speed.

Surgical Repair for Anomalous Origin of the Right Coronary Artery from the Pulmonary Artery.

Anomalous origin of the right coronary artery from the pulmonary artery (ARCAPA) is a very rare congenital heart defect. Herein, we describe three cases of ARCAPA in an 8 months old, 18 months old, and 4 year old child. Two cases were incidentally diagnosed using a computed tomographic angiograph, and the other was incidentally diagnosed using a coronary angiograph. These cases underwent a reimplantation technique on diagnosis and resulting in positive clinical outcomes during the follow-up period which was a mean of 1.5 years.