Burt S. Tilley

The Olin curriculum: thinking toward the future

In 1997, the F. W. Olin Foundation of New York established the Franklin W. Olin College of Engineering, Needham, MA, with the mission of creating an engineering school for the 21st century. Over the last five years, the college has transformed from an idea to a functioning entity that admitted its first freshman class in fall 2002. This paper describes the broad outlines of the Olin curriculum with some emphasis on the electrical and computer engineering degree. The curriculum incorporates the best practices from many other institutions as well as new ideas and approaches in an attempt to address the future of engineering education.

Electrokinetic Instabilities in Thin Microchannels

An important class of electrokinetic, microfluidic devices aims to pump and control electrolyte working liquids that have spatial gradients in conductivity. These high-gradient flows can become unstable under the application of a sufficiently strong electric field. In many of these designs, flow channels are thin in the direction orthogonal to the main flow and the conductivity gradient. Viscous stresses due to the presence of these walls introduce a stabilizing force that plays a major role in determining the overall instability. A thin channel model for fluid flow is developed and shown to provide good agreement with a complete three-dimensional model for channel aspect ratios ≲0.1.

On Undercompressive Shocks and Flooding in Countercurrent Two-Layer Flows

We consider the countercurrent flow of two incompressible immiscible viscous fluids in an inclined channel. This configuration may lead to the phenomena of ‘flooding’, i.e. the transition from a countercurrent regime to a cocurrent regime. This transition is marked by a variety of transient behaviour, such as the development of large-amplitude waves that impede the flow of one of the fluids to the reversal of the flow of the denser fluid. From a lubrication approximation based on the ratio of the channel height to the downstream disturbance wavelength, we derive a nonlinear system of evolution equations that govern the interfacial shape separating the two fluids and the leading-order pressure. This system, which assumes fluids with disparate density and dynamic viscosity ratios, includes the effects of viscosity stratification, inertia, shear and capillarity. Since the experimental constraints for this effective system are unclear, we consider two ways to drive the flow: either by fixing the volumetric flow rate of the gas phase or by fixing the total pressure drop over a downstream length of the channel. The latter forcing results in a single evolution equation whose dynamics depends non-locally on the interfacial shape. From both of these driven systems, admissible criteria for Lax shocks, undercompressive shocks and rarefaction waves are investigated. These criteria, through a numerical verification, do not depend significantly on the inertial effects within the more dense layer. The choice of the local/non-local constraints appears to play a role in the transient growth of undercompressive shocks, and may relate to the phenomena observed near the onset of flooding.

Thermocapillary Control of Rupture in Thin Viscous Fluid Sheets

We consider the evolution of a thin viscous fluid sheet subject to thermocapillary effects. Using a lubrication approximation we find, for symmetric interfacial deflections, coupled evolution equations for the interfacial profile, the streamwise component of the fluid velocity and the temperature variation along the surface. Initial temperature profiles change the initial flow field through Marangoni-induced shear stresses. These changes then lead to preferred conditions for rupture prescribed by the initial temperature distribution. We show that the time to rupture may be minimized by varying the phase difference between the initial velocity profile and the initial temperature profile. For sufficiently large temperature differences, the phase difference between the initial velocity and temperature profiles determines the rupture location.

Instabilities and Taylor Dispersion in Isothermal Binary Thin Fluid Films

Experiments with glycerol-water thin films flowing down an inclined plane reveal a localized instability that is primarily three dimensional. These transient structures, referred to as “dimples,” appear initially as nearly isotropic depressions on the interface. A linear stability analysis of a binary mixture model in which barodiffusive effects dominate over thermophoresis (i.e., the Soret effect) reveals unstable modes when the components of the mixture have different bulk densities and surface tensions. This instability occurs when Fickian diffusion and Taylor dispersion effects are small, and is driven by solutalcapillary stresses arising from gradients in concentration of one component, across the depth of the film. Qualitative comparison between the experiments and the linear stability results over a wide range of parameters is presented.

Spiraling Into Gallstone Disease: A Physicist’s Spin

Scientists have been fascinated for decades with the ability of nature to form self‐assembled structures of various configurations. One such configuration is a spiral or a helix. The (double) helical geometrical configuration is a well‐known secondary structure of DNA; however, DNA is not the only biological system possessing this shape. Spirals have been found in a variety of biological and synthetic systems, one of which is bile in the gallbladder. In this system, helical ribbons appear as metastable intermediates in the process of cholesterol crystallization that leads to the formation of gallstones. The bile system is particularly interesting and unique due to the richness in variety of the helical structures formed. Understanding the mechanisms for nature’s self‐assembly of helical ribbons is crucial in both the prevention of gallstone disease and in developing potential technological and medical applications. We describe a model bile system, composed of three major components of native bile in water...