Manipulation of Spherical Droplets on a Liquid Platform Using Thermal Gradients

In the recent years, there has been a growing interest in droplet-based (digital) microfluidics for which, reliable means of droplet manipulation are required. In this study we demonstrate thermal actuation of droplets on liquid platforms, which is ideal for biochemical microsystems and lab-on-chip applications because droplets can be transported with high speed, good control and minimal thermal loading as compared to using conventional solid substrates. In addition, other disadvantages of using solid surfaces such as evaporation, contamination, pinning, hysteresis and irreversibility of droplet motion are avoided. Based on the theoretical development and measurements, a silicon-based droplet transportation platform was developed with embedded Titanium micro heaters. A shallow liquid pool of inert liquid (FC-43) served as the carrier liquid. Heaters were interfaced with control electronics and driven through a computer graphical user interface. By creating appropriate spatio-temporal thermal gradient maps, transport of droplets on predetermined pathways was successfully demonstrated with high level of robustness, speed and reliability. The video shows normal imaging of droplet manipulation accompanied by the corresponding infrared thermal imaging showing the spatio-temporal temperature maps and the outline of the drop as it moves towards hot spots.Comment: 63rd APS - Division of Fluid Dynamics - 201

Microscale resin-bonded permanent magnets for magnetic micro-electro-mechanical systems applications

A micromachining technique has been developed for the fabrication of microscale resin-bonded permanent magnets. Magnetic paste has been prepared from Sr-ferrite powder and an epoxy resin, filled into lithographically defined molds, and formed into resin-bonded magnets after room temperature curing. Coercivity of 356 kA/m (4480 Oe), retentivity of 33 mT (330 G), and energy density of 2.7 kJ/m(3) have been achieved in 65-mum-thick disk arrays with lateral dimensions ranging from 50 to 200 mum. Based on the developed magnet, a magnetic MEMS actuator has been designed, fabricated, and characterized. Actuation current up to +/-60 mA operated the actuator up to 70 mum in attractive and repulsive motion. This work can be used for producing thick-film type permanent magnets, which can be scaled from a few tens of micrometers to millimeters on various substrates

Droplets on liquid surfaces: Dual equilibrium states and their energy barrier

Floating aqueous droplets were formed at oil-air interface, and two stable configurations of (i) non-coalescent droplet and (ii) cap/bead droplet were observed. General solutions for energy and force analysis were obtained for both configurations and were shown to be in good agreement with the experimental observations. The energy barrier obtained for transition from configuration (i) to configuration (ii) was correlated to the droplet release height and the probability of non-coalescent droplet formation

Effect of laminar velocity profile variation on mixing in microfluidic devices: The sigma micromixer

The effect of the laminar velocity profile and its variation on mixing phenomena at the reduced scale is studied. It is shown that the diffusive mass flux between two miscible streams, flowing laminar in a microchannel, is enhanced if the velocity at their diffusion interface is increased. Based on this idea, an in-plane passive micromixing concept is proposed and implemented in a working device (sigma micromixer). This mixer shows considerable mixing performance by periodically varying the flow velocity profile, such that the maximum of the profile coincides with the transversely progressing diffusion fronts repeatedly throughout the mixing channel

Discrete Droplet Manipulation on Liquid Platforms using Thermal Gradients

AbstractManipulation of droplet motion on the free surface of an liquid platform using thermal gradient has been studied. Two distinct motion mechanisms were observed. Droplets that remain spherical are propelled from cold to warm region and those which break up into a sessile lens move in the opposite direction. Driving forces acting on the droplet due to interfacial-tension gradients arising from induced thermal gradients were analyzed using theoretical models. By adopting a liquid platform, limitations associated with previously-reported capillary-based mechanisms, which used chemical/thermally-patterned solid substrates, are overcome to make the proposed scheme suitable for droplet-based bio-chemical lab on chips: minimal droplet temperature fluctuation (<5°C); avoiding droplet-pinning and contact-angle hysteresis allow for higher transport speeds (~4 mm/sec) and no evaporative losses or contaminations

Droplet actuation on a liquid layer due to thermocapillary motion: Shape effect

In the thermocapillary migration of droplets on the free surface of immiscible liquids, we observe that the lens-shaped drops move from warm toward cooler regions while spherical drops move in the opposite direction. We explain this dual behavior using an analysis of surface deformation and velocity profiles of thin liquid layers subject to a lateral thermal gradient. Liquid platforms allow thermocapillary transport of drops with higher migration speeds than solid substrates and lower internal temperature fluctuation. Such conditions may be exploited in biochemical microsystems where droplet evaporation, contamination, and surface pinning need to be avoided

An analytical model for the wettability switching characteristic of a nanostructured thermoresponsive surface

The applications of thermoresponsive surfaces require the development of a rigorous mathematical treatment for these surfaces to understand and improve their behavior. We propose an analytical model to describe the transfer characteristics (variation in contact angle versus temperature) of a unique nanostructured thermosensitive surface, consisting of silica nanoparticles and a hydrophilic/hydrophobic thermoresponsive polymer, poly(N-isopropylacrylamide). Three different thermo-sensitive platforms were fabricated and the contact angle change of a water droplet on the surface with varying surface temperature was analytically modeled

Effect of ultraviolet radiation exposure on room-temperature hydrogen sensitivity of nanocrystalline doped tin oxide sensor incorporated into microelectromechanical systems device

The effect of ultraviolet (UV) radiation exposure on the room-temperature hydrogen (H-2) sensitivity of nanocrystalline indium oxide (In2O3)-doped tin oxide (SnO2) thin-film gas sensor is investigated in this article. The present sensor is incorporated into microelectromechanical systems device using sol-gel dip-coating technique. The present sensor exhibits a very high sensitivity, as high as 65 000-110 000, at room temperature, for 900 ppm of H-2 under the dynamic test condition without UV exposure. The H-2 sensitivity is, however, observed to reduce to 200 under UV radiation, which is contrary to the literature data, where an enhanced room-temperature gas sensitivity has been reported under UV radiation. The observed phenomenon is attributed to the reduced surface coverage by the chemisorbed oxygen ions under UV radiation, which is in consonance with the prediction of the constitutive equation, proposed recently by the authors, for the gas sensitivity of nanocrystalline semiconductor oxide thin-film sensors

Hydrogen-discriminating nanocrystalline doped-tin-oxide room-temperature microsensor

Highly hydrogen (H-2)-selective [relative to carbon monoxide (CO)] sensor, operating at room temperature, has been fabricated using the micronanointegration approach involving the deposition of the nanocrystalline indium oxide (In2O3)-doped tin oxide (SnO2) thin film on microelectromechanical systems device. The present microsensor exhibits high room-temperature sensitivity towards H-2 (S=12 700); however, it is insensitive to CO at room temperature. In view of the different gas selectivity mechanisms proposed in the literature, it is deduced that the In2O3 doping, the presence of InSn4 phase, the low operating temperature (room temperature), the mesostructure, the small sizes of H-2 and H2O molecules, the bulky intermediate and final reaction products for CO, and the electrode placement at the bottom are the critical parameters, which significantly contribute to the high room-temperature H-2 selectivity of the present microsensor over CO. The constitutive equation for the gas sensitivity of the semiconductor oxide thin-film sensor, proposed recently by the authors, has been modified to qualitatively explain the observed H-2 selectivity behavior

A Small Molecule Inhibitor of ITK and RLK Impairs Th1 Differentiation and Prevents Colitis Disease Progression

In T cells, the Tec kinases IL-2-inducible T cell kinase (ITK) and resting lymphocyte kinase (RLK) are activated by TCR stimulation and are required for optimal downstream signaling. Studies of CD4(+) T cells from Itk(-/-) and Itk(-/-)Rlk(-/-) mice have indicated differential roles of ITK and RLK in Th1, Th2, and Th17 differentiation and cytokine production. However, these findings are confounded by the complex T cell developmental defects in these mice. In this study, we examine the consequences of ITK and RLK inhibition using a highly selective and potent small molecule covalent inhibitor PRN694. In vitro Th polarization experiments indicate that PRN694 is a potent inhibitor of Th1 and Th17 differentiation and cytokine production. Using a T cell adoptive transfer model of colitis, we find that in vivo administration of PRN694 markedly reduces disease progression, T cell infiltration into the intestinal lamina propria, and IFN-gamma production by colitogenic CD4(+) T cells. Consistent with these findings, Th1 and Th17 cells differentiated in the presence of PRN694 show reduced P-selectin binding and impaired migration to CXCL11 and CCL20, respectively. Taken together, these data indicate that ITK plus RLK inhibition may have therapeutic potential in Th1-mediated inflammatory diseases