Bau, Haim H.

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  • Publication
    Carbon nanopipettes for cell probes and intracellular injection
    (2008-01-09) Schrlau, Michael G; Falls, Erica M; Ziober, Barry L; Bau, Haim H
    We developed integrated, carbon-based pipettes with nanoscale dimensions (CNP) that can probe cells with minimal intrusion, inject fluids into the cells, and concurrently carry out electrical measurements. Our manufacturing technique does not require cumbersome nanoassembly and is amenable to mass production. Using CNPs, we demonstrate the injection of reagents into cells with minimal intrusion and without inhibiting cell growth.
  • Publication
    Confinement and Manipulation of Actin Filaments by Electric Fields
    (2007-08-01) Purohit, Prashant K; Arsenault, Mark E; Goodman, Yale E; Zhao, Hui; Bau, Haim H
    When an AC electric field was applied across a small gap between two metal electrodes elevated above a surface, rhodamine-phalloidin-labeled actin filaments were attracted to the gap and became suspended between the two electrodes. The variance 〈s2(x)〉 of each filament's horizontal, lateral displacement was measured as a function of electric field intensity and position along the filament. 〈s2(x)〉 markedly decreased as the electric field intensity increased. Hypothesizing that the electric field induces tension in the filament, we estimated the tension using a linear, Brownian dynamic model. Our experimental method provides a novel means for trapping and manipulating biological filaments and for probing the surface conductance and mechanical properties of single polymers.
  • Publication
    In Situ Liquid-Cell Electron Microscopy of Colloid Aggregation and Growth Dynamics
    (2011-06-16) Grogan, Joseph M.; Bau, Haim; Rotkina, Lolita
    We report on real-time observations of the aggregation of gold nanoparticles using a custom-made liquid cell that allows for in situ electron microscopy. Process kinetics and fractal dimension of the aggregates are consistent with three-dimensional cluster-cluster diffusion-limited aggregation, even for large aggregates, for which confinement effects are expected. This apparent paradox was resolved through in situ observations of the interactions between individual particles as well as clusters at various stages of the aggregation process that yielded the large aggregates. The liquid cell described herein facilitates real-time observations of various processes in liquid media with the high resolution of the electron microscope.
  • Publication
    Ceramic Microchips for Capillary Electrophoresis-electrochemistry
    (1999-07-01) Henry, Charles S.; Zhong, Min; Lunte, Susan M.; Bau, Haim; Kim, Moon; Santiago, Jorge J.
    A capillary electrophoresis–electrochemistry chip constructed from low-temperature co-fired ceramic (LTCC) tape is presented. This is the first report of such a chip constructed in this manner using these materials. Electroosmotic flow at pH 7 is demonstrated by the migration of a neutral marker, catechol. The separation and detection of two phenolic compounds are presented.
  • Publication
    A mathematical model of lateral flow bioreactions applied to sandwich assays
    (2003-05-22) Qian, Shizhi; Bau, Haim H
    Lateral flow (LF) bio-detectors facilitate low-cost, rapid identification of various analytes at the point of care. The LF cell consists of a porous membrane containing immobilized ligands at various locations. Through the action of capillary forces, samples and reporter particles are transported to the ligand sites. The LF membrane is then scanned or probed, and the concentration of reporter particles is measured. A mathematical model for sandwich assays is constructed and used to study the performance of the LF device under various operating conditions. The model provides insights into certain experimental observations including the reduction in the level of the detected signal at high target analyte concentrations. Furthermore, the model can be used to test rapidly and inexpensively various operating conditions, assist in the device's design, and optimize the performance of the LF device.
  • Publication
    Magneto hydrodynamic (MHD) pump fabricated with ceramic tapes
    (2002-01-31) Zhong, Jihua; Yi, Mingqiang; Bau, Haim H
    The use of Magneto Hydro Dynamics (MHD) to circulate fluids in conduits fabricated with low temperature co-fired ceramic tapes is described. Conduits shaped like toroidal and rectangular loops were fabricated. Electrodes printed on the ceramic substrate along the conduits' walls facilitated transmission of electric currents through the test fluids. When the devices were subjected to a magnetic field, the resulting Lorentz forces propelled the liquids. The paper details the fabrication process and describes experiments with mercury slugs, saline solution, and deionized water. The measured fluid velocities were compared with theoretical predictions.
  • Publication
    Flow Patterns and Reaction Rate Estimation of RedOx Electrolyte in the Presence of Natural Convection
    (2006-10-01) Qian, Shizhi; Chen, Zongyuan; Wang, Jing; Bau, Haim H
    Transport processes in an upright, concentric, annular, electrochemical reactor filled with RedOx electrolyte solution are studied experimentally and theoretically. The electrodes form the two vertical surfaces of the reactor. The theoretical calculations consist of the solution of the Navier-Stokes and the Nernst-Planck equations accounting for species' diffusion, migration, convection, and electrochemical reactions on the electrodes' surfaces as a function of the difference in the electrodes' potentials and the average concentration of the electrolyte. Since the convection is driven by density gradients, the momentum and mass transport equations are strongly coupled. In spite of the small dimensions (mm-scale) of the reactor, the current transmitted through the electrolyte is significantly enhanced by natural convection. The current is measured as a function of the difference in the electrodes' potentials. To obtain the reaction rate constants, an inverse problem is solved and the reaction rate constants are determined by minimizing the discrepancy between theoretical predictions and experimental observations. As an example, we study the reversible electrochemical reaction Fe++++e- = Fe++ on platinum electrodes. The paper demonstrates that natural convection plays a significant role even when the reactor’s dimensions are on the millimeter scale and that it is possible to predict reaction rate constants while accounting for significant mass transfer effects.
  • Publication
    On the Stability and Flow Reversal of an Asymmetrically Heated Open Convection Loop
    (1981) Bau, Haim H.; Torrance, Kenneth E.
    Experimental results are reported for a U-shaped, free convection loop. The top of the loop is open to an isothermal reservoir. The horizontal leg and one vertical leg are heated at rates Q1 and Q2, respectively. The loop is filled either with water or a watersaturated porous medium. Symmetric heating and asymmetric heating favouring the ascending leg of the loop both yield stable flows. Asymmetric heating favouring the descending leg leads to stable flows when the ratio Q1/Q2 is above a critical value. Below this critical value, the flow is observed to oscillate with increasing amplitude until the direction of flow in the loop undergoes a reversal. A steady flow follows the reversal. Analytical results include a stability analysis and time-dependent, one-dimensional numerical calculations, both of which compare favourably with experiment.
  • Publication
    Complex magnetohydrodynamic low-Reynolds-number flows
    (2003-07-01) Xiang, Yu; Bau, Haim H
    The interaction between electric currents and a magnetic field is used to produce body (Lorentz) forces in electrolyte solutions. By appropriate patterning of the electrodes, one can conveniently control the direction and magnitude of the electric currents and induce spatially and temporally complicated flow patterns. This capability is useful, not only for fundamental flow studies, but also for inducing fluid flow and stirring in minute devices in which the incorporation of moving components may be difficult. This paper focuses on a theoretical and experimental study of magnetohydrodynamic flows in a conduit with a rectangular cross section. The conduit is equipped with individually controlled electrodes uniformly spaced at a pitch L. The electrodes are aligned transversely to the conduit's axis. The entire device is subjected to a uniform magnetic field. The electrodes are divided into two groups A and C in such a way that there is an electrode of group C between any two electrodes of group A. We denote the various A and C electrodes with subscripts, i.e., Ai and Ci , where i = 0, ±1, ±2, ... . When positive and negative potentials are, respectively, applied to the even and odd numbered A electrodes, opposing electric currents are induced on the right and left hand sides of each A electrode. These currents generate transverse forces that drive cellular convection in the conduit. We refer to the resulting flow pattern as A. When electrodes of group C are activated, a similar flow pattern results, albeit shifted in space. We refer to this flow pattern as C. By alternating periodically between patterns A and C, one induces Lagrangian chaos. Such chaotic advection may be beneficial for stirring fluids, particularly in microfluidic devices. Since the flow patterns A and C are shifted in space, they also provide a mechanism for Lagrangian drift that allows net migration of passive tracers along the conduit's length.
  • Publication
    Experiments on the stabilization of the no-motion state of a fluid layer heated from below and cooled from above
    (1997-07-28) Tang, Jie; Bau, Haim H
    It is demonstrated experimentally that through the use of feedback control, it is possible to stabilize the no-motion (conductive) state of a fluid layer confined in a circular cylinder heated from below and cooled from above (the Rayleigh-BĂ©nard problem), thereby postponing the transition from a no-motion state to cellular convection. The control system utilizes multiple sensors and actuators. The actuators consist of individually controlled heaters microfabricated on a silicon wafer which forms the bottom of the test cell. The sensors are diodes installed at the fluid's midheight. The sensors monitor the deviation of the fluid temperatures from preset, desired values and direct the actuators to act in such a way as to eliminate these deviations.