Our group's research interests are in fluid mechanics and transport processes, with a special interest in problems at the interface between continuum mechanics and statistical mechanics. One area of research concerns fundamental studies of complex fluids. Complex fluids is a generic label for materials that are composed of microstructural elements that interact via colloidal, hydrodynamic, and Brownian forces. Familiar examples of such fluids are suspensions, colloidal dispersions, liquid crystals, ferrofluids, electrorheological fluids, and polymer solutions and melts.

marco AT marco-heinen DOT de

My research centers around complex fluids of charged Brownian particles. The systems that I am particularly interested in comprise suspensions of charged colloidal particles, protein solutions, and complex (dusty) plasmas. By means of semi-analytical physico-chemical theories and computer simulations, I study colloidal charge regulation, fluid phase particle correlations, phase behavior, rheology and diffusion on various length scales. Among the theoretical and computer simulations tools that I am using are liquid integral equations, density functional theory, analytical approximations for many-body hydrodynamic interactions, and Stokesian Dynamics simulations.

# Eric Burkholder

My project focuses on the rheology of active suspensions – colloidal suspensions whose constituent particles are able to undergo directed motion through internally generated forces (e.g. bacterial suspensions, or suspensions of nanoparticles that move via an asymmetric chemical reaction at the particle surface). In particular, we are interested in the microscopic viscoelastic and diffusive properties of the suspension under the influence of external perturbations. Using analytical dilute theory, numerical schemes, and dynamic simulation methods, we will probe these micromechanical properties in order to i) improve our fundamental understanding of the nonequilibrium behavior of active suspensions, ii) give insight as to how one might exploit the unique properties of swimmers in material design, and iii) understand the role of activity in biological processes, such as cell lysis.

# Isaac Fees

My work focuses on understanding the behavior of electrolyte solutions in confined geometries to develop strategies that exploit surface properties for sustainable energy devices. I am specifically interested in how to devise a nanotube to control the mixing of fresh water and salt water masses to capture the released free energy as an electric current.

# Kevin Marshall

My research interests can be categorized under the fields of probability and stochastic processes, mesh-free Lagrangian particle simulation methods (e.g. SPH, DPD, SDPD), and Stokesian and Brownian dynamics. I am interested in applying ideas inherent in these mesh-free particle methods to colloidal dynamics problems. Moreover I am interested in developing new, efficient, and perhaps hybrid types of these methods and algorithms that are computationally efficient and are capable of accurately capturing all of the interesting dynamics of a problem. Recently I have coded various simulations on CUDA-enabled GPUs. A problem that I am currently working on looks at the effects of hydrodynamic interactions on macromolecular motion in jammed or high volume fraction systems (e.g. cellular environments) where irregular particle shapes and other field interactions exist. My hobbies include casual graphics programming, developing factor model quant simulations, playing golf, mountain biking, and triathlon multisport.

# Mikey Phan

My project explores the hydrodynamic behavior of systems involving self-propelled particles in confined environments. To that end, we use Stokesian dynamics to help analyze these systems and gain a better fundamental understanding of their tendencies to self-assemble and phase-separate. When I’m not playing around with these particles, you can usually find me either playing disc golf (rather poorly), singing, or playing the piano.

# Charlie Slominski

My primary project is determining the microscopic transport and mechanical properties of hydrogels. Hydrogels are a fluid-like material that find use in biomimetic solar cells, synthetic trees for soil remediation, sustained drug delivery vectors, and of course many gelatinous desserts! Hydrogels are generally formed by a cross-linked polymer microstructure immersed in water. While hydrogels have been the subject of many macroscale rheological studies, few have sought to characterize their microrheological properties. Using Brownian and Stokesian Dynamics simulations, I am investigating properties like hindered diffusion and stress-strain relationships in hydrogels, and in particular the combined effects of thermal motion and microstructural rigidity on those properties.

# Sho Takatori

My project analyzes the motion of self-propelled particles. Microorganisms like E. coli alternate between two modes: during the "run," it swims straight towards a given orientation. It then "tumbles" into a new random direction by unbundling its flagella, perhaps due to a collision with another body. By switching between the modes, the swimmer undergoes a random-walk process with a net migration towards a favorable environment at long times. This run-and-tumble feature allows us to model the motion as a random-walk process, since the detailed reorientation mechanism of the micro-swimmer is not important. We use both Brownian dynamics simulations and dilute analytical theory to analyze the nonequilibrium behavior of active suspensions.

My project focuses on the dynamical behaviors of complex fluids in confined geometries. Confined complex fluids are frequently encountered in micro- and nano-fluidic devices, micro-rheology, porous materials, and transport within biological cells. For suspensions, confinement not only alters the short-time behaviors such as high frequency dynamical viscosity and short-time self-diffusivities, but also introduces long-time changes that influence collective morphology, long-time diffusivity and rheology, etc. With the newly developed Stokesian Dynamics framework for confined suspensions and using both theoretical and computational tools, the goal of the project is to achieve a fundamental understanding of these confined systems.

# Current Undergraduates

# Recent Post Doctoral Scholars

# Recent Undergraduates

- Moriah Bischann
* Caltech*
- Dan Jacobson
* Brown University*
- Eli Sorey
* Caltech*
- Anirban Ghosh
* Johns Hopkins University*
- Christian Aponte Rivera
* University of Puerto Rico, Mayaguez*
- Greg Rubinstein
* Princeton University*
- Kevin Gu
* Caltech*