Bau, Haim H.
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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 HIt 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-Bénard 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.Publication Carbon Nanopipettes Characterize Calcium Release Pathways in Breast Cancer Cells(2008-07-03) Schrlau, Michael G; Brailiou, Eugen; Patel, Sandip; Gogotsi, Yury; Dun, Nae J; Bau, Haim HCarbon-based nanoprobes are attractive for minimally-invasive cell interrogation but their application in cell physiology has thus far been limited. We have developed carbon nanopipettes (CNPs) with nanoscopic tips and used them to inject calcium-mobilizing messengers into cells without compromising cell viability. We identify pathways sensitive to cyclic adenosine diphosphate ribose (cADPr) and nicotinic acid adenine dinucleotide phosphate (NAADP) in breast carcinoma cells. Our findings demonstrate the superior utility of CNPs for intracellular delivery of impermeant molecules and, more generally, for cell physiology studies. The CNPs do not appear to cause any lasting damage to cells. Their advantages over the commonly used glass pipettes include smaller size, breakage and clogging resistance, and potential for multifunctionality such as concurrent injection and electrical measurements.Publication Kelvin-Helmhotz Instability for Parallel Flow in Porous Media: A Linear Theory(1982) Bau, Haim H.Two fluid layers in fully-saturated porous media are considered. The lighter fluid is above the heavier one so that in the absence of motion the arrangement is stable and the interface is flat. It is shown that when the fluids are moving parallel to each other at different velocities, the interface may become unstable (the Kelvin-Helmholtz instability). The corresponding conditions for marginal stability are derived for Darcian and non-Darcian flows. In both cases, the velocities should exceed some critical values in order for the instability to manifest itself. In the case of Darcy's flow, however, an additional condition, involving the fluids' viscosity and density ratios, is required.Publication Torsional Sensor Applications in Two-Phase Fluids(1993-09-01) Kim, Jin O.; Bau, Haim H.; Liu, Yi; Lynnworth, Lawrence C.; Lynnworth, Steven A.; Hall, Kimberly A.; Jacobson, Saul A.; Korba, James. A.; Murphy, Robert J.; Strauch, Michael A.; King, Kyle G.A solid corrosion-resistant torsional waveguide of diamond cross section has been developed to sense on-line and in real-time the characteristics of the liquid in which it is submerged. The sensor can measure, among other things, the liquid content of a bubbly medium; the density of adjacent pure liquids; the equivalent density of liquid-vapor mixtures or particulate suspensions; a suspension's concentration; and the liquid level. The sensor exploits the phenomenon that the speed of propagation of a torsional stress wave in a submerged waveguide with a noncircular cross section is inversely proportional to the equivalent density of the liquid in which the waveguide is submerged. The sensor may be used to conduct measurements along distances ranging from 20 mm to 20 m and over a wide range of temperatures and pressures, e.g., from the cryogenic temperature of liquid nitrogen, -196°C, up to hot pressurized water at 300°C and 7 MPa. A self-calibrating three-zone sensor and associated electronics have also been developed to compensate for any sensor inaccuracies due to operation over a wide range of temperature. In some of the water experiments at room temperature, unexpected attenuation of the guided torsional waves was observed. This excess attenuation depends in part on the waveguide's surface finish. It appears to be caused by air microbubbles adhering to the waveguide, imposing one of the practical limits on the maximum sensor length in nondegassed or aerated water.Publication Analysis of sedimentation biodetectors(2005-05-01) Qian, Shizhi; Bürger, Raimund; Bau, Haim HA bead-based sedimentation biodetector is studied theoretically. The biodetector operates with a suspension of settling beads, non-settling reporters, and target analytes – all initially suspended in a buffer solution. The reporters can be either fluorescent molecules or small particles. The functionalized beads interact with the reporters and target analytes while settling under the action of gravitational, electric, and/or magnetic fields. Both sandwich and competitive assays with hindered settling are considered. In the sandwich format, in the presence of target analytes, the reporters bind to the beads and settle (the target analytes provide the link between the beads and the reporters). A reduction in the reporters' concentration indicates the presence of target analytes. In the competitive format, both target analytes and reporters compete for bead-based binding sites. In the absence of target analytes, one would observe a reduction in the suspended reporters' concentration. The model allows one to predict the reporters' concentration in solution as a function of initial bead, reporter, and target analyte concentrations and provides a means for the reactor's optimization.Publication Magneto-hydrodynamic Stirrer for Stationary and Moving Fluids(2005-05-13) Qian, Shizhi; Bau, Haim HA magneto-hydrodynamic (MHD) stirrer that exhibits chaotic advection is designed, modeled, and tested. The stirrer can operate as a stand-alone component or it can be incorporated into a MHD-controlled network. The stirrer consists of a conduit equipped with individually controlled electrodes positioned along its opposing walls. The conduit is filled with an electrolyte solution and positioned in a uniform magnetic field. When a potential difference is applied across pairs or groups of electrodes, the resulting current interacts with the magnetic field to induce Lorentz forces and fluid motion. When the potential difference is applied across opposing electrodes that face each other, the fluid is propelled along the conduit’s length. When the potential difference is applied across diagonally positioned electrodes, a circulatory motion results. When the potential difference alternates periodically across two or more such configurations, chaotic motion evolves and efficient mixing is obtained. This device can serve as both a stirrer and a pump. The advantage of this device over previous designs of MHD stirrers is that it does not require electrodes positioned away from the conduit’s walls. Since this device has no moving parts, the concept is especially suitable for microfluidic applications.Publication The Effect of an Adjacent Viscous Fluid on the Transmission of Torsional Stress Waves in a Submerged Waveguide(1991-03-01) Kim, Jin O.; Wang, Yuzhou; Bau, Haim H.The effects of an adjacent fluid's viscosity and density on the characteristics of torsional stress waves transmitted in a waveguide with a circular cross section are studied theoretically and experimentally. Expressions for the torsional waves speed, dispersion relations, and attenuation are obtained as functions of the adjacent fluid's viscosity and density. The theoretical results are compared with experimental observations. It is demonstrated that a devices similar to the one described herein can be used as a rugged, real-time, on-lines sensor for measuring the viscosity of a fluid with a known density. Such a sensor can measure the viscosity of fluids with a density viscosity product (ρfμ) greater than 100kg2/m4s to a precision of 1% or betterPublication A magnetohydrodynamic chaotic stirrer(2002-10-08) Yi, Mingqiang; Qian, Shizhi; Bau, Haim HA magnetohydrodynamic (MHD) stirrer that exhibits chaotic advection is investigated experimentally and theoretically. The stirrer consists of a circular cavity with an electrode (C) deposited around its periphery. Two additional electrodes (A) and (B) are deposited eccentrically inside the cavity on the bottom. The cavity is positioned in a uniform magnetic field that is parallel to the cylinder's axis, and it is filled with a weak electrolyte solution. Fluid motion is induced in the cavity by applying a potential difference across a pair of electrodes. A closed-form, analytical solution is derived for the MHD creeping flow field in the gap between the two eccentric cylinders. A singular solution is obtained for the special case when the size of the inner electrode shrinks to a point. Subsequently, passive tracers' trajectories are computed when the electric potential differences are applied alternately across electrodes AC and BC with period T. At small periods T, the flow is regular and periodic in most of the cavity. As the period increases, so does the complexity of the motion. At relatively large periods, the passive tracer experiences global chaotic advection. Such a device can serve as an efficient stirrer. Since this device has no moving parts, it is especially suitable for microfluidic applications. This is yet another practical example of a modulated, two-dimensional Stokes flow that exhibits chaotic advection.Publication Instrument for Simultaneous Measurement of Density and Viscosity(1989-02-07) Kim, Jin O.; Bau, Haim H.The speed of torsional stress waves transmitted in solid waveguides submerged in a liquid depends, among other things, on the liquid's density and viscosity and the waveguides' cross-sectional geometry. By measuring the speed of torsional stress waves in two waveguides of different cross-sectional geometries, one can obtain both the liquid's density and viscosity. An online, real-time sensor for the simultaneous measurement of density and viscosity is described. The article details the sensor's principles of operation and reports experimental results conducted using viscosity standard calibration liquids with wen-known thermophysical properties. For fluids with density ρf > 1 X 103 kg/m3 , it is estimated that the instrument can measure density with a precision better than 0.5%. For fluids with the product shear viscosity (µ) and density, ρfµ> 100 kg2/(m4s), it can measure the shear viscosity with a precision better than 1%.Publication A magneto-hydrodynamically controlled fluidic network(2003-01-15) Bau, Haim H; Zhu, Jianzhong; Qian, Shizhi; Zhiang, YuThe paper describes fluidic networks consisting of individually controlled branches. The networks' basic building blocks are conduits equipped with two electrodes positioned on opposing walls. The entire device is either subjected to an external uniform magnetic field or fabricated within a magnetic material. When a prescribed potential difference is applied across each electrode pair, it induces current in the liquid (assumed to be at least a weak electrolyte solution). Analogously with electric circuits, by judicious application of the potential differences at various branches, one can direct liquid flow in any desired way without a need for mechanical pumps or valves. Equipped with additional, internally located electrodes, the network branches double as stirrers capable of generating chaotic advection. The paper describes the basic building blocks for such a network, the operation of these branches as stirrers, a general linear graph-based theory for the analysis and optimal control of fluidic magneto-hydrodynamic networks, an example of a network fabricated with low temperature, co-fired ceramic tapes, and preliminary experimental observations that illustrate that the ideas described in this paper can, indeed, be implemented in practice.