Bau, Haim H.
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PublicationTorsional 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. PublicationCarbon 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. PublicationExperiments 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. PublicationKelvin-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. PublicationLimitations of Linear Control of Thermal Convection in a Porous Medium(2006-07-01) Zhao, Hui; Bau, Haim HThe ability of linear controllers to stabilize the conduction (no-motion) state of a saturated porous layer heated from below and cooled from above is studied theoretically. Proportional, suboptimal robust(H∝) and linear quadratic Gaussian (H2) controllers are considered. The proportional controller increases the critical Rayleigh number for the onset of convection by as much as a factor of 2. Both the H2 and H∝ controllers stabilize the linearized system at all Rayleigh numbers. Although all these controllers successfully render negative the real part of the linearized system’s eigenvalues, the linear operator of the controlled system is non-normal and disturbances undergo substantial growth prior to their eventual, asymptotic decay. The dynamics of the nonlinear system are examined as a function of the disturbance’s amplitude when the system is subjected to the "most dangerous disturbances." These computations provide the critical amplitude of the initial conditions above which the system can no longer be stabilized. This critical amplitude decreases as the Rayleigh number increases. To facilitate extensive computations, we examine two-dimensional convection in a box containing a saturated porous medium, heated from below and cooled from above, as a model system. The heating is provided by a large number of individually controlled heaters. The system’s state is estimated by measuring the temperature distribution at the box’s midheight. All the controllers considered here render the linearized, controlled system’s operator non-normal. The transient amplification of disturbances limits the "basin of attraction" of the nonlinear system’s controlled state. By appropriate selection of a controller, one can minimize, but not eliminate, the controlled, linear system’s non-normality. PublicationA stirrer for magnetohydrodynamically controlled minute fluidic networks(2002-09-05) Qian, Shizhi; Zhu, Jianzhong; Bau, Haim HMagnetohydrodynamics may potentially provide a convenient means for controlling fluid flow and stirring fluids in minute fluidic networks. The branches of such fluidic networks consist of conduits with rectangular cross sections. Each conduit has two individually controlled electrodes positioned along opposing walls and additional disk-shaped electrodes deposited in the conduit's interior away from its sidewalls. The network is positioned in a uniform magnetic field. When one applies a potential difference between a disk-shaped electrode and two wall electrodes acting in tandem, circulatory motion is induced in the conduit. When the potential difference alternates periodically across two or more such configurations, complicated (chaotic) motions evolve. As the period of alternation increases, so does the complexity of the flow. We derive a two-dimensional, time-independent expression for the magnetohydrodynamic creeping flow around a centrally positioned disk-shaped electrode in the limit of zero radius. With the aid of this expression, the trajectories of passive tracers are computed as functions of the alternations protocol and the electrodes' locations. The theoretical results are qualitatively compared with flow visualization experiments. PublicationOn the translation of a cylinder in a long tube(2004-03-08) Bau, Haim H; Liu, Hui; Hu, Howard HWe study the motion of a cylindrical particle translating slowly in a long tube as a function of the particle's dimensions and placement both in the presence and the absence of external pressure gradients. The cylinder acts as a "leaky" piston, generating both fluid recirculation and through flow. When the particle is long, analytic expressions are obtained for both the velocity field and the force needed to sustain the particle’s motion as functions of the particle's position and dimensions. When the particle is short, a superposition-based algorithm is outlined to facilitate economical numerical calculations. When the particle is placed off center in the tube, torque will act on the particle. When the particle is unguided, this torque will preclude coaxial motion and cause the particle to follow an oscillatory trajectory. PublicationThe dielectrophoresis of cylindrical and spherical particles submerged in shells and in semi-infinite media(2004-03-15) Liu, Hui; Bau, Haim HThe dielectrophoretic forces acting on and the resulting velocities of cylindrical and spherical particles embedded in perfectly dielectric viscous fluids are calculated analytically. The fluids are confined in cylindrical/spherical shells and in semi-infinite media with prescribed potential distributions along the surfaces of the media. The forces are calculated by evaluating the Maxwell stress tensor. The velocities of the particles are obtained by solving the Stokes equation for creeping flow. The range of validity of force calculations based on the dipole-moment approximation is estimated. PublicationPoint Detection of Pathogens in Oral Samples(2005-06-01) Malamud, Daniel; Bau, Haim H; Niedbala, S.; Corstjens, Paul L.A.M.We have outlined our progress with respect to developing a novel device for monitoring oral samples for bacterial and/or viral pathogens. The system is based on an existing device for measuring drugs of abuse in an oral sample. The sample is collected on an absorbent pad that delivers a metered dose to the cassette. The sample is then separated into 4 channels for the detection of antigen, RNA or DNA, and host antibodies to the pathogen. The detection system involves the Upconverting Phosphor Technology (UPT), whereby the captured pathogen analyte is detected by interrogation of the UPT particles with near-infrared light, and the emitted visible light is detected by the analyzer. Several of the steps in this process have already been worked out for viral and/or bacterial pathogens, and most of the remaining effort will be aimed at integrating these steps into a single microfluidic device while maintaining the current sensitivity. PublicationMicrofluidic chaotic stirrer utilizing induced-charge electro-osmosis(2007-06-28) Zhao, Hui; Bau, Haim HRecently, there has been a growing interest in using induced electro-osmosis to pump fluids in microfluidic devices. We show that induced electro-osmosis can also be used to promote stirring and chaotic advection. To this end, we study theoretically a stirrer in which the flow patterns are alternated in time. We first analyze an idealized embodiment of the stirrer that admits a simple analytical solution for the flow patterns. The stirrer consists of a concentric annulus whose outer surface is defined by an array of electrodes that provide a spatially varying potential distribution. The resulting electric field induces quadruple electro-osmotic flow around the inner cylinder. By timewise alternating the potential distribution around the outer cylinder, we induce chaotic advection in the cavity. Subsequently, we carry out numerical simulations for a more realistic design that can be readily constructed, and demonstrate that it is possible to induce chaotic advection also in this case.