My research focuses on the use of externally applied fields for guiding colloidal particles, fluids, chemical species and biological cells into configurations useful for materials synthesis and biological applications. Gradients in electric potential, temperature, concentration, and shear stress are examples of driving forces that are externally tunable. Frequently these driving forces have non-intuitive consequences on systems with boundaries which themselves contain gradients either imposed or modulated by the external field. The goal of my research is to elucidate the physics underlying the interactions between externally applied and internally mediated fields in technologically relevant systems in materials science and bioengineering, using a combination of experiments, theory and numerical modeling. This poster summarizes six ongoing research projects:

[1] Electrohydrodynamic Aggregation and Segregation of Unilamellar Vesicles

[2] Enzymatic Reactions in Microfluidic Devices

[3] Influence of Shear Stress on ATP Release from Red Blood Cells

[4] Effects of Substrate Thermal Conductivity on Flow in Evaporating Droplets

[5] Non-coalescence of Oppositely Charged Droplets

[6] Electrically Driven Flow on Inhomogeneous Electrodes

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[1] Electrohydrodynamic Aggregation and Size Segregation of Unilamellar Vesicles (Collaborators: S. Lecuyer, O. Vincent & H.A. Stone)

We demonstrate that electrohydrodynamic (EHD) flow induces polydisperse suspensions of unilamellar vesicles to aggregate laterally along electrodes. Upon aggregation, smaller vesicles move underneath the larger vesicles, gradually lifting the larger vesicles off of the electrode entirely. We demonstrate that this phenomenon provides a method for gently separating giant unilamellar vesicles from polydisperse suspensions.

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[2] Enzymatic Reactions in Microfluidic Devices (Collaborators: J. Wan & H.A. Stone)

We establish simple scaling laws for enzymatic reactions in microfluidic devices, and we demonstrate that kinetic parameters obtained conventionally using multiple stop-flow experiments may instead be extracted from a single microfluidic experiment. Introduction of an enzyme and substrate species in different arms of a Y-shaped channel allows the two species to diffuse across parallel streamlines and to begin reacting. For reactions that follow standard Michaelis-Menten kinetics, the power law takes the form P ~ (V_max / K_m) x^5/2. Numerical simulations and experiments using the reaction between luciferase and ATP as a model system are both shown to accord with the model.

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[3] Influence of Shear Stress on ATP Release from Red Blood Cells (Collaborators: J. Wan, C. Best, R. Horton, G. Guidotti, E. H. Abraham & H. A. Stone)

To understand the influence of shear stress on the kinetics of adenosine triphosphate (ATP) release from red blood cells (RBCs), we mimic arterial constrictions using a series of channels in microfluidic devices. We show that the amount of released ATP increases roughly exponentially with the magnitude of the shear stress, but that there is a critical duration of stress (>2 ms) required for RBCs to release significant amounts of ATP. The results suggest that RBCs are sensitive not only to the diameter of arterial constrictions but to their length, an effect with important physiological and medical implications.

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[4] Effects of Substrate Thermal Conductivity on Flow in Evaporating Droplets (Collaborators: P.G. Kim, C. Domingues, J. Wan & H. A Stone)

Non-uniform evaporation from sessile droplets gives rise to radial convection within the drop, yielding the well-known coffee stain pattern. Any non-uniformity in the evaporative flux, however, also induces a gradient in temperature and consequently a gradient in surface tension, yielding a Marangoni flow. Here we report the existence of a critical substrate thermal conductivity below which the direction of the Marangoni flow reverses, thereby affecting the particle deposition pattern.

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[5] Non-coalescence of Oppositely Charged Droplets (Collaborators: A. Belmonte, F. Dollar, D. A. Weitz & H. A. Stone)

We demonstrate the existence of a critical electric field strength above which oppositely charged drops do not coalesce. Application of an external electric field causes appropriately positioned and oppositely charged drops to migrate toward one another. Upon contact, surface tension acts to pull the drops together. For low applied field strengths, the oppositely charged drops coalesce, but at higher field strengths the drops are repelled from one another after contact. Qualitatively, the drops appear to ‘bounce' off one another. We derive a critical field strength for bouncing based on a competition between the time scales for charge transfer and the action of surface tension. The results have broad implications for applications where charged drops are manipulated by electric fields, including microfluidics, atmospheric science, and electrospray ionization.

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[6] Electrically Driven Flow on Inhomogeneous Electrodes (Collaborators: P. Jiang, M. Slowik, I. A. Aksay, D. A. Saville)

Current density inhomogeneities on electrodes – of physical, chemical, or optical origin – induce long-range electrohydrodynamic (EHD) fluid motion directed toward the regions of higher current density. Here we analyze the flow and its implications for the orderly arrangement of colloidal particles on patterned electrodes. A scaling analysis indicates that the EHD flow velocity is proportional to the product of the applied voltage and the difference in current density between adjacent regions on the electrode. Exact analytical solutions for the EHD streamlines are derived for the case of a spatially oscillatory perturbation in current density along the electrode. We demonstrate that proper placement of scratches on an electrode yields arbitrary patterns of colloidal particles.