Eric Brown

Postdoctoral Scholar
James Franck Institute, The University of Chicago
office: GCIS E025
email:  embrown@uchicago.edu
phone: (773) 702-6075
Curriculum Vitae

Research

My research is in the fields of experimental soft condensed matter and fluid physics. I am interested in understanding the mechanics of complex systems in the natural world such as turbulent flows, complex fluids, and granular materials. These systems are ubiquitous in our everyday lives, but their mechanics can be challenging to understand because there are many particles which may have non-linear interactions and/or may be out of equilibrium.

Areas of current research include:

• Large-scale flow dynamics in turbulence
• Shear thickening suspensions
• Mechanics of jammed granular matter

Large-scale flows in turbulence

Turbulent flows often spontaneously form large-scale flow structures (comparable to the system size), which can be seen in the wind, ocean currents, industrial cooling towers, or a pot of water on the stove. These large-scale flows can play a major role in heat transport, making them important to industrial and geophysical processes. However, very little is known about the physics of how these large-scale flows form and what drives their dynamics in these turbulent systems.

To understand such large-scale flows, I performed experiments on turbulent Ralyeigh-Benard convection in which water in a cylindrical container was heated from below to generate a bouyancy-driven flow. The dynamics of the large-scale flow include spontaneous changes in the orientation of the flow [Brown et al., PRL (2005), Brown & Ahlers, J. Fluid. Mech (2006)], and slow rotation of the flow orientation due to the Coriolis force [Brown & Ahlers, Phys. Fluids (2006)]. I made a model using stochastic differential equations which was motivated by the Navier-Stokes equations to describe these flow dynamics [Brown & Ahlers, PRL (2007), Brown & Ahlers, Phys. Fluids (2008a)]. In this model, the large-scale flow is driven by buoyancy which is balanced by viscous drag, and turbulent fluctuations areby a represented by a stochastic driving force which can drive orientation changes. Perturbations due to the Coriolis force and other asymmetries are included in this model [Brown & Ahlers, Phys. Fluids (2008a)]. This model also describes the twisting and sloshing oscillation modes observed in these flows when a pressure forcing from the sidewall is included [Brown & Ahlers, J. Fluid Mech. (2009)].

Currently I am developing a series of experiments to generalize this model to include other effects such as different container shapes to make the models applicable to large-scale flows in geophysical systems such as ocean circulations.

Another focus of my previous experiments in turbulent Rayleigh-Benard convection was to make high-resolution measurements of the heat transport to test the Grossman-Lohse scaling model [Nikolaenko et al., J. Fluid Mech. (2005), Funfschilling et al., J. Fluid Mech. (2005)]. This included corrections due to non-Boussinesq effects (variation of fluid properties other than density with temperature)[Ahlers et al., J. Fluid Mech. (2006)], the conductivity of the boundaries [Brown et al., Phys. Fluids (2005)], and tilting the sample [Ahlers et al., J. Fluid Mech. (2005)]. I also made measurements to characterize the temperature profiles in the bulk [Brown & Ahlers, Europhysics Letters (2007)], the flow circulation frequency [Brown et al., J. Stat. Mech. (2008)], and a twist oscillation mode of the circulation [Funfschilling et al., J. Fluid Mech. (2008)].

Shear thickening suspensions

Complex fluids can have a variety of useful properties. One of the most interesting is shear thickening, for example in suspensions of cornstarch in water. When the suspension is stirred weakly, it is thin, but when stirred harder it gets dramatically thicker, and when the stirring stops it becomes a thin fluid again. This property is useful for the development of shock absorbing or damping materials. While shear thickening occurs in many densely packed suspensions or colloids, the physical mechanism had not been previously known.

I performed experiments that show these dense suspensions dilate (expand) under shear to push against the boundary, leading to frictional contacts between particles that produce the large stresses observed in shear thickening [Brown & Jaeger (2011, preprint)]. The confining stress is usually due to surface tension at the liquid-air interface, but can also come from the stiffness of the walls for enclosed flows. In the latter case, even dry granular materials shear thicken. I also showed the onset of the shear thickening regime is generally set by a stress scale corresponding to particle interactions which prevent shear and dilation, whether those interactions come from chemical interactions, an electrostatic potential, induced electric and magnetic fields, or gravity [Brown et al., Nature: Materials (2010),(supplementary material)]. I found that shear thickening is controlled by a critical point that coincides with the jamming transition, where grains are packed just tightly enough to form a rigid structure [Brown & Jaeger, PRL (2009), Brown et al. (2011, preprint)]. I have also shown that shear thickening can be found in systems as thin as 2 particle layers [Brown et al., J. Rheology (2010)].

Currently, I am developing experiments to explore the dynamical response of shear thickening systems, as well to understand the nature of dilation in complex fluids.

Mechanics of jammed granular matter

The strength of a collection of granular matter is very important issue for the soil under the foundation of a building. I have been investigating different applications for the unique mechanics of granular material by controlling boundary conditions or the connectivity between grains.

One area I worked on in collaboration with iRobot Corporation is to develop soft robots that consist of granular materials. While robots are traditionally built with hard, specialized components, nature is full of creatures that are mostly soft and can robustly handle many tasks. We developed a new approach to universal gripping using a bag full of granular material attached to an arm. When pushed onto a target object, the granular material flows around and forms to the shape of the target. When air is vacuumed out of the bag, the grains jam into a solid and contract slightly to squeeze and hold the target. This gripper can handle a wide variety of objects with different weights, shapes, and fragility with a single control mode eliminating the need for complicated control systems required for robotic hands with metal fingers. We showed the holding force comes from a combination of friction, suction, and interlocking mechanism, and related the contribution of each mechanism to the strength of the jammed grains [Brown et al., PNAS (2010),(supplementary material)].
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