Haim H. Bau

Gas flow in micro-channels

An experimental and theoretical investigation of low Reynolds number, high subsonic Mach number, compressible gas flow in channels is presented. Nitrogen, helium, and argon gases were used. The channels were microfabricated on silicon wafers and were typically 100 μm wide, 10 4 μm long, and ranged in depth from 0.5 to 20 μm. The Knudsen number ranged from 10 -3 to 0.4. The measured friction factor was in good agreement with theoretical predictions assuming isothermal, locally fully developed, first-order, slip flow.

A minute magneto hydro dynamic (MHD) mixer

Abstract A theoretical and experimental investigation of a magneto hydrodynamic (MHD) stirrer is presented. Such a stirrer can be used to enhance mixing in micrototal analysis systems. The stirrer utilizes arrays of electrodes deposited on a conduit’s walls. The conduit is filled with an electrolyte solution. By applying alternating potential differences across pairs of electrodes, currents are induced in various directions in the solution. In the presence of a magnetic field, the coupling between the magnetic and electric fields induces body (Lorentz) forces in the fluid. Since, the electrodes can be patterned in various ways, fairly complex flow fields can be generated. In particular, in this paper, we describe the induction of cellular motion. This motion can be used to deform and stretch material interfaces and to enhance mixing. The MHD stirrer does not utilize any moving parts. The experimental observations are in good agreement with theoretical predictions.

Liquid Transport In Micron And Submicron Channels

There has been a growth of interest in fluid transport in very small structures. The basis for this interest derives from the application of micromachining technology to problems in fluidics. Several aspects of this problem are reviewed and discussed including some of our recent research on this topic. The problems discussed may be separated into those dealing with biological systems and those that explore the applicability of the macroscopic Navier-Stokes equations to very small planar channels. In the work conducted at the University of Pennsylvania, an experimental investigation of fluid flow in extremely small channels was conducted. Three devices have been constructed with channels of rectangular cross-section ranging in area from 7200 to 80 square microns. It was found that in the relatively large flow channels that the experimental observations were in rough agreement with the predictions from the Navier-Stokes equations. However, in the smallest of the chan-nels, there was a significant deviation from the Navier-Stokes predictions.

Liquid transport in micron and submicron channels

Abstract An experimental investigation of fluid flow in extremely small channels is presented. Potential applications for such channels include cooling of electronic circuits, and reactors for modification and separation of biological cells. The immediate goal is to determine at what length scales the continuum assumptions break down and if the Navier-Stokes equations adequately predict fluid behavior. In order to accomplish this, experiments are being conducted in progressively thinner flow channels. We have constructed three channels of rectangular cross-section ranging in area from 7200 to 80 square microns, utilizing the recent advances in microfabrication. In this paper, preliminary results are reported pertaining to friction measurements. It is found that in the relatively large flow channels the experimental observations are in rough agreement with the predictions from the Navier-Stokes equations. However, in the smallest of the channels, there is a significant deviation from the Navier-Stokes predictions.

An integrated, self-contained microfluidic cassette for isolation, amplification, and detection of nucleic acids

A self-contained, integrated, disposable, sample-to-answer, polycarbonate microfluidic cassette for nucleic acid—based detection of pathogens at the point of care was designed, constructed, and tested. The cassette comprises on-chip sample lysis, nucleic acid isolation, enzymatic amplification (polymerase chain reaction and, when needed, reverse transcription), amplicon labeling, and detection. On-chip pouches and valves facilitate fluid flow control. All the liquids and dry reagents needed for the various reactions are pre-stored in the cassette. The liquid reagents are stored in flexible pouches formed on the chip surface. Dry (RT-)PCR reagents are pre-stored in the thermal cycling, reaction chamber. The process operations include sample introduction; lysis of cells and viruses; solid-phase extraction, concentration, and purification of nucleic acids from the lysate; elution of the nucleic acids into a thermal cycling chamber and mixing with pre-stored (RT-)PCR dry reagents; thermal cycling; and detection. The PCR amplicons are labeled with digoxigenin and biotin and transmitted onto a lateral flow strip, where the target analytes bind to a test line consisting of immobilized avidin-D. The immobilized nucleic acids are labeled with up-converting phosphor (UCP) reporter particles. The operation of the cassette is automatically controlled by an analyzer that provides pouch and valve actuation with electrical motors and heating for the thermal cycling. The functionality of the device is demonstrated by detecting the presence of bacterial B.Cereus, viral armored RNA HIV, and HIV I virus in saliva samples. The cassette and actuator described here can be used to detect other diseases as well as the presence of bacterial and viral pathogens in the water supply and other fluids.

A disposable microfluidic cassette for DNA amplification and detection

A pneumatically driven, disposable, microfluidic cassette comprised of a polymerase chain reaction (PCR) thermal cycler, an incubation chamber to label PCR amplicons with up-converting phosphor (UPT) reporter particles, conduits, temperature-activated, normally closed hydrogel valves, and a lateral flow strip, was constructed and tested. The hydrogel valves, which were opened and closed with the aid of electrically controlled thermoelectric units, provided a simple means to seal the PCR reactor and suppress bubble formation. The hydrogel-based flow control was electronically addressable, leakage-free, and biocompatible. To test the device, a solution laden with genomic DNA isolated from B. cereus was introduced into the microfluidic cassette and a specific 305 bp fragment was amplified. The PCR amplicons were labelled with the phosphor (UPT) reporter particles, applied to the lateral flow strip, bound to pre-immobilized ligands, and detected with an IR laser that scanned the lateral flow strip and excited the phosphor (UPT) particles that, in turn, emitted light in the visible spectrum. The UPT particles do not bleach, they provide a permanent record, and they readily facilitate the filtering of background noise. The cassette described herein will be used for rapid testing at the point of care.

Controlling chaos in a thermal convection loop

It is demonstrated experimentally and theoretically that through the use of an active (feedback) controller one can dramatically modify the nature of the flow in a toroidal thermal convection loop heated from below and cooled from above. In particular, we show how a simple control strategy can be used to suppress (laminarize) the naturally occurring chaotic motion or induce chaos in otherwise time-independent flow. The control strategy consists of sensing the deviation of fluid temperatures from desired values at a number of locations inside the loop and then altering the wall heating to either counteract or enhance such deviations.

Boiling in low-permeability porous materials

Abstract Experimental observations are reported on boiling in a vertical circular cylinder heated from below and cooled from above. The cylinder is rilled with low-permeability, water-saturated porous materials. With the onset of boiling an almost isothermal, 2-phase (wet steam) zone forms near the bottom. This zone is overlain by a liquid layer with an essentially linear vertical temperature gradient. The temperature field in the overlying liquid layer is oscillatory whenever the height of the 2-phase zone exceeds a critical value. A simple 1-dim. model is used to predict the height of the 2-phase zone, the dry-out heat flux, and a necessary condition for the formation of a 2-phase zone.

The Nanoaquarium: A Platform for In Situ Transmission Electron Microscopy in Liquid Media

Transmission electron microscopes (TEMs) and scanning transmission electron microscopes (STEMs) are powerful tools for imaging on the nanoscale. These microscopes cannot be typically used to image processes taking place in liquid media because liquid simply evaporates in the high-vacuum environment of the microscope. In order to view a liquid sample, it is thus necessary to confine the liquid in a sealed vessel to prevent evaporation. Additionally, the liquid layer must be very thin to minimize electron scattering by the suspending medium. To address these issues, we have developed a flow cell with a height of tens of nanometers, sandwiched between two thin silicon nitride membranes. The cell is equipped with electrodes for actuation and sensing. The cell is thin enough to allow the transmission of electrons and the real-time imaging of nanoparticles suspended in liquid. This paper details the fabrication process, which relies on plasma-activated wafer bonding. Some of the advantages of our nanoaquarium include the thinnest observation chamber of any reported in situ TEM/STEM device, integrated electrodes for sensing and actuation, and wafer-scale processing that allows bulk device production. Device performance was demonstrated by STEM imaging of gold and polystyrene nanoparticles suspended in water with excellent resolution. Potential applications of the device include imaging of colloidal crystal formation, aggregation, nanowire growth, electrochemical deposition, and biological interactions.

An isothermal amplification reactor with an integrated isolation membrane for point-of-care detection of infectious diseases

A simple, point of care, inexpensive, disposable cassette for the detection of nucleic acids extracted from pathogens was designed, constructed, and tested. The cassette utilizes a single reaction chamber for isothermal amplification of nucleic acids. The chamber is equipped with an integrated, flow-through, Flinders Technology Associates (Whatman FTA®) membrane for the isolation, concentration, and purification of DNA and/or RNA. The nucleic acids captured by the membrane are used directly as templates for amplification without elution, thus simplifying the cassettes flow control. The FTA membrane also serves another critical role-enabling the removal of inhibitors that dramatically reduce detection sensitivity. Thermal control is provided with a thin film heater external to the cassette. The amplification process was monitored in real time with a portable, compact fluorescent reader. The utility of the integrated, single-chamber cassette was demonstrated by detecting the presence of HIV-1 in oral fluids. The HIV RNA was reverse transcribed and subjected to loop-mediated, isothermal amplification (LAMP). A detection limit of less than 10 HIV particles was demonstrated. The cassette is particularly suitable for resource poor regions, where funds and trained personnel are in short supply. The cassette can be readily modified to detect nucleic acids associated with other pathogens borne in saliva, urine, and other body fluids as well as in water and food.