Session HK: Fluidic Devices

Valveless Impedance Pumping: Scaling and Viscous Effects

Valveless pumping through the periodic compression of a pliant tube with geometric asymmetry was first noted by Liebau in 1954. Recent studies by Hickerson and Gharib (J. Fluid Mech. 2006) and Avrahami and Gharib (J. Fluid Mech. 2008) highlight the role of resonance in generating flow in valveless impedance pumps. The pump has also been shown to perform down to scales of at least 250 $\mu$m by Rinderknecht, Hickerson, and Gharib (J. Micromech. Microeng. 2005). The impedance pump holds great promise for micro-scale applications such as lab-on-chip and biomedical devices where it could encounter a wide range of fluid viscosities. A thorough investigation of the pump under viscosity scaling is a critical step in understanding its potential. We experimentally investigated a 2 mm diameter pump in open and closed loop configurations with a fluid viscosity range of 1-50 cS and excitation frequency range of 1-150 Hz. This corresponds to a Womersley Number range of $\alpha$ = 0.3-30. The pump showed robust performance with increasing viscosity. The closed loop system showed flow rates as high as 20 mL/min corresponding to Reynolds Number up to Re = 300.   

Nano-Channel Impedance Arrays for Biomolecular Detection

We have designed and tested nanochannel impedance sensors for biomolecular detection based on fundamental analyses of the underlying electrokinetic phenomena. Probe-functionalized nanocolloids (macroions) with specific hybridized and unhybridized impedance signals are used to capture multiple molecular targets. These nanocolloids are driven electrophoretically, electro-osmotically or dielectrophoretically by a slow (and high-amplitude) AC field into the nanochannels, where field focusing amplifies their impedance signal. Detection is carried out at a higher frequency close to the inverse RC time of the channel. We find, however, that the impedance of a multi-channel array is not a superposition of the single nanochannels, once the depletion/enrichment zones at the two ends of the nanochannels overlap. Hence, detection sensitivity can be greatly enhanced if the nonlinear and non-equilibrium ion and macro-ion accumulation dynamics in the nanochannel is understood.   

Free-surface microfluidics for detection of airborne explosives

A novel microfluidic, remote-sensing, chemical detection platform has been developed for real-time sensing of airborne agents. The key enabling technology is a newly developed concept termed Free-Surface Fluidics (FSF), where one or more fluidic surfaces of a microchannel flow are confined by surface tension and exposed to the surrounding atmosphere. The result is a unique open channel flow environment that is driven by pressure through surface tension, and not subject to body forces, such as gravity. Evaporation and flow rates are controlled by microchannel geometry, surface chemistry and precisely-controlled temperature profiles. The free-surface fluidic architecture is combined with Surface-Enhanced Raman Spectroscopy (SERS) to allow for real-time profiling of atmospheric species and detection of airborne agents. The aggregation of SERS nanoparticles is controlled using microfluidics, to obtain dimer nanoparticle clusters at known streamwise positions in the microchannel. These dimers form SERS hot-spots, which amplify the Raman signal by 8 -- 10 orders of magnitude. Results indicate that explosive agents such as DNT, TNT, RDX, TATP and picric acid in the surrounding atmosphere can be readily detected by the SERS system. Due to the amplification of the SERS system, explosive molecules with concentrations of parts per trillion can be detected, even in the presence of interferent molecules having six orders of magnitude higher concentration.   

On-chip isothermal, chemical cycling polymerase chain reaction (ccPCR)

We demonstrate a novel ccPCR technique for microfluidic DNA amplification where temperature is held constant in space and time. The polymerase chain reaction is a platform of choice for biological assays and typically based on a three-step thermal cycling: DNA denaturation, primers annealing and extension by an enzyme. We here demonstrate a novel technique where high concentration chemical denaturants (solvents) denature DNA. We leverage the high electrophoretic mobility of DNA and the electrical neutrality of denaturants to achieve chemical cycling. We focus DNA with isotachophoresis (ITP); a robust electrophoretic preconcentration technique which generates strong electric field gradients and protects the sample from dispersion. We apply a pressure-driven flow to balance electromigration velocity and keep the DNA sample stationary in a microchannel. We drive the DNA through a series of high denaturant concentration zones. DNA denatures at high denaturant concentration. At low denaturant concentration, the enzyme creates complementary strands. DNA reaction kinetics are slower than buffer reactions involved in ITP. We demonstrate successful ccPCR amplification for detection of E. Coli. The ccPCR has the potential for simpler chemistry than traditional PCR.   

Titanium based flat heat pipes for computer chip cooling

We are developing a highly conductive flat heat pipe (called Thermal Ground Plane or TGP) for cooling computer chips. Conventional heat pipes have circular cross sections and thus can't make good contact with chip surface. The flatness of our TGP will enable conformal contact with the chip surface and thus enhance cooling efficiency. Another limiting factor in conventional heat pipes is the capillary flow of the working fluid through a wick structure. In order to overcome this limitation we have created a highly porous wick structure on a flat titanium substrate by using micro fabrication technology. We first etch titanium to create very tall micro pillars with a diameter of 5 $\mu $m, a height of 40 $\mu $m and a pitch of 10 $\mu $m. We then grow a very fine nano structured titania (NST) hairs on all surfaces of the pillars by oxidation in H$_{2}$0$_{2}$. In this way we achieve a wick structure which utilizes multiple length scales to yield high performance wicking of water. It's capable of wicking water at an average velocity of 1 cm/s over a distance of several cm. A titanium cavity is laser-welded onto the wicking substrate and a small quantity of water is hermetically sealed inside the cavity to achieve a TGP. The thermal conductivity of our preliminary TGP was measured to be 350 W/m-K, but has the potential to be several orders of magnitude higher.   

Manipulation of microfluidic drops with laser patterns: Stationary and non stationary effects

Control over individual microfluidic drops can be achieved through Marangoni flows when a thermal gradient is created along the drop surface through laser heating. The maximum force induced by the Marangoni force is measured at $F_m \approx 200$ nN. A drop advancing in a microchannel can be blocked if $F_m$ is higher than the drag from the external flow and if the transit time of the interface through the laser waist ($\tau$) is longer than $\tau_m = 5$ ms. Velocity and temperature field measurements show that the formation time of the thermocapillary flow correlates with $\tau$ and that $\tau_m$ is determined by the time required to establish the thermal gradient. The production of complex laser patterns extends the limits of the technique: Stationary spatial laser patterning with holographic techniques can be used to stop faster drops, since the heated zone is larger. On the other hand, non stationary patterns obtained with scanning mirrors can produce the same blocking with a lower mean power.   

Energy dissipation in a fluidic nanomechanical resonator

The fluid-structure interaction of resonating microcantilevers in fluid has been widely studied and is a cornerstone in nanomechanical sensor development. Operation in fluid environments presents significant challenges due to the strong enhancement of fluid damping effects with miniaturization. Recently, Burg et al. [Nature, Vol. 446, 1066 (2007)] proposed a new type of microcantilever device whereby a microfluidic channel was embedded inside the cantilever, which resulted in unprecedented sensitivity. We study the fluid dynamics of these devices by presenting a theoretical model and experimental measurements. Significantly, it is found that energy dissipation in these devices is not a monotonic function of fluid viscosity. A direct consequence is that miniaturization does not necessarily result in degradation in the quality factor, which may indeed be enhanced. This highly desirable feature is unprecedented in current nanomechanical devices and permits direct miniaturization to enhance sensitivity in liquid environments.   

Dynamics of viscous droplets in microfluidic cross-flows

Cross-flow injections are used to experimentally investigate the dynamic response of individual viscous droplets to a sharp increase of their velocity in a square microchannel. As a basis for diluting emulsions on-chip, a train of droplets is subjected to an additional injection of the continuous phase from symmetric side-channels. The local modification of the flow velocity produces a broad range of dynamics, including the formation of unstable slender viscous structures and intriguing 'spoon-like' droplets. Deformations and relaxation times are examined as a function of flow and fluids properties with a particular emphasis on the break-up conditions of high-viscosity droplets.   

Motion of beads in an oscillatory rotating fluid: micro-bead-beating

One method for mechanical lysis of biological cells and spores is to mix them with a suspension of beads and vigorously ``shake'' the mixture. The precise mechanisms of lysis are not understood but lysis is thought to result from collisions between the beads and the cells and the associated stresses exerted on the cells. For instance, in the micro-bead-beater$^{TM}$ instrument from Claremont BioSolutions LLC (Upland, CA), the ``shaking'' occurs when a small cartridge filled with a mixture of cells/spores and 100-micron beads is driven at high frequencies in a small arc trajectory. In this presentation, we describe our initial modeling effort aimed at understanding this system via analysis of the trajectories of beads within such an instrument. The equations governing the motion of non-neutrally-buoyant spherical beads in an oscillatory rotating flow are derived and analyzed numerically. The resulting trajectories are found to be quite complex and very different from those in a steadily rotating fluid. A catalog of possible trajectories at various values of the governing dimensionless parameters is presented.   

Electrokinetic trapping of biomolecules in a microdroplet outside micro/nanochannel hybrid system

We describe a new method of electrokinetic trapping process in a droplet-micro/nanochannel hybrid system based on concentration polarization (CP) phenomena near nanochannels. Two microchannels are connected to the outside of the PDMS chip and micordroplet connects the channels at the outside of the PDMS. A nanojunction connects a third, ground channel and the droplet. With DC bias, CP is initiated and ions start to be depleted at the anodic side, with EOF towards the droplet. With tangential electric field through the droplet, any charged species would form a plug inside the droplet and EOF, depending on the voltage configuration of each anodic side, would define the droplet volume. By having small droplet volume with plug accumulating sample in it, highly concentrated droplet can be formed. Since the droplet is formed outside the channels, these accumulated molecules can be directly dispensed to a sample plate and be detected using conventional immunoassays and other techniques without further diffusion.