Generation of high-resolution surface temperature distributions

J. Microelectromech. Syst.

Troian³

et al. 2003

215

167

Abstract-We have designed a microfluidic device for the actuation of liquid droplets or continuous streams on a solid surface by means of integrated microheater arrays. The microheaters provide control of the surface temperature distribution with high spatial resolution. These temperature gradients locally alter the surface tension along droplets and thin films thus propelling the liquid toward the colder regions. In combination with liquophilic and liquophobic chemical surface patterning, this device can be used as a logistic platform for the parallel and automated routing, mixing and reacting of a multitude of liquid samples, including alkanes, poly(ethylene glycol) and water.[1024]

Section: Electronic Control Setupmentioning

confidence: 99%

“…In these experiments, the stripes were located directly above the heating resistors. By varying the dc-voltage applied to the th resistor, the heating power and hence the local temperature can be set with a time-constant entirely determined by the thickness and thermal properties of the substrate [19]. Fig.…”

Section: Device Fabricationmentioning

confidence: 99%

Thermocapillary actuation of droplets on chemically patterned surfaces by programmable microheater arrays

Darhuber¹,

J. Microelectromech. Syst.

Troian³

et al. 2003

215

167

“…The power can be reduced by at least one order of magnitude by using a polymeric substrate of low thermal conductivity instead of glass. 22 Battery operation of this microfluidic device is, therefore, practicable.…”

Section: Dϭ10mentioning

confidence: 99%

“…As the presence of an overlying liquid film affects the thermal response time of the resistors, they can be used as position sensors providing feedback control. 22 This device can also serve as a chemical sensor since the liquid is in continuous contact with the ambient vapor phase. The variety of tasks that can be accomplished with highresolution thermal maps should, therefore, inspire alternative designs for pocket-sized diagnostic devices.…”

Section: Dϭ10mentioning

confidence: 99%

Microfluidic actuation by modulation of surface stresses

Darhuber

Applied Physics Letters

Davis

et al. 2003

Self Cite

165

147

We demonstrate the active manipulation of nanoliter liquid samples on the surface of a glass or silicon substrate by combining chemical surface patterning with electronically addressable microheater arrays. Hydrophilic lanes designate the possible routes for liquid migration while activation of specific heater elements determines the trajectories. The induced temperature fields spatially modulate the liquid surface tension thereby providing electronic control over the direction, timing, and flow rate of continuous streams or discrete drops. Temperature maps can be programed to move, split, trap, and mix ultrasmall volumes without mechanically moving parts and with low operating voltages of 2-3 V. This method of fluidic actuation allows direct accessibility to liquid samples for handling and diagnostic purposes and provides an attractive platform for palm-sized and battery-powered analysis and synthesis. © 2003 American Institute of Physics. ͓DOI: 10.1063/1.1537512͔Miniaturized automated systems for liquid routing, mixing, analysis, and synthesis are rapidly expanding diagnostic capabilities in medicine, genomic research, and material science.1 Liquid flow in microchannels can be regulated by pressure gradients, 2 thermocapillary pumping, 3 electrokinetic forces, 4,5 or magnetohydrodynamic pumping.6,7Electrowetting 8,9 and dielectrophoresis 10 have also been used to move droplets on an open surface. These techniques typically require high operating voltage and high electrolyte concentrations. We demonstrate a different method for liquid handling and transport that uses programmable surface temperature distributions, in conjunction with chemical substrate patterning, to provide electronic control over the direction, timing, and flow rate. This method capitalizes on the large surface-to-volume ratio inherent in microscale systems since the gas-liquid and liquid-solid surface energies are modulated to induce and confine flow. It works equally well with polar or nonpolar liquids, requires no mechanically moving parts and operates at very low voltages. The open architecture is best suited to liquids of low volatility. Encapsulation schemes that retain the free liquid-air interfaces can minimize evaporative loss.Local heating of a liquid film at a position x reduces the surface tension ␥(x) to produce a thermocapillary shear stress •n ϭ"␥ϭ(‫ץ/␥ץ‬T)"T that pulls liquid toward regions of cooler surface temperature T. [11][12][13] Since ‫‪T‬ץ/␥ץ‬ is essentially temperature independent for all liquids, the driving force and flow direction are proportional to "T. For a thin flat liquid layer, the average flow speed and flow rate ͑per unit width͒ are given by (x)ϭh(x,t) •n /2(x) and Q(x,t)ϭh(x,t)(x,t), where h(x,t) is the film thickness and (x) the local viscosity. This phenomenon forms the basis of our microfluidic device for actuating continuous streams and discrete droplets.The sample layout 14 for driving continuous streams is shown in Fig. 1͑a͒. Hydrophilic stripes connect pairs of 4.5-mm-wide square reservoir pads; the...

“…Our laboratory has previously demonstrated two methods 10,11 for droplet detection and sensing that make use of the coplanar microelectrode arrays used for thermocapillary actuation. [7][8][9] The first method 10 monitors the rise time of embedded microheaters, from which can be extracted droplet location, volume, or composition due to changes in thermal conductivity.…”

mentioning

confidence: 99%

Microfluidic detection and analysis by integration of thermocapillary actuation with a thin-film optical waveguide

Applied Physics Letters

Troian

Wagner

2005

Self Cite

We demonstrate a nonintrusive optical method for microfluidic detection and analysis based on evanescent wave sensing. The device consists of a planar thin-film waveguide integrated with a microfluidic chip for directed surface flow. Microliter droplets are electronically transported and positioned over the waveguide surface by thermocapillary actuation. The attenuated intensity of propagating modes is used to detect droplet location, to monitor dye concentration in aqueous solutions, and to measure reaction rates with increasing surface temperature for a chromogenic biochemical assay. This study illustrates a few of the capabilities possible by direct integration of optical sensing with surface-directed fluidic devices. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1922075͔ Microfluidic devices offer tremendous potential for electrical, optical, and spectroscopic interrogation of nanoliter liquid samples. Devices based on directed surface flow induced by electrowetting, 1-4 dielectrophoresis, 5,6 or thermocapillary actuation 7-9 permit reconfigurable instruction sets for liquid positioning and mixing sequences on glass or silicon substrates. What requires further development are in situ sensor arrays for autonomous manipulation of sample location, volume, temperature, speed, and composition. Direct integration of miniaturized sensors enables application of the feedback control necessary for automated use of labon-a-chip technologies.Our laboratory has previously demonstrated two methods 10,11 for droplet detection and sensing that make use of the coplanar microelectrode arrays used for thermocapillary actuation. 7-9 The first method 10 monitors the rise time of embedded microheaters, from which can be extracted droplet location, volume, or composition due to changes in thermal conductivity. The second method records the capacitance change induced by an overlying droplet. The rapid response for electrode widths comparable to the liquid film thickness allows detection of droplet position, volume, composition, and evaporative loss for nanoliter samples. These sensing techniques, which probe samples by thermal or electric fields, are not suitable for all applications. Integrated optical and spectroscopic probes, such as evanescent wave sensing ͑EWS͒, offer a number of advantages in this respect. Previous studies of EWS have used external pumps to generate a continuous stream of liquid through flow cells 12-15 that contact and cover the entire beam path. However, hermetically sealed channels 16,17 placed against a waveguide surface produce large coupling losses and small signal-to-noise ratios. Interferometric analysis and measurement of coupling angles also increases device complexity. 18,19 In this paper we demonstrate detection and analysis of discrete droplets or rivulets that are in direct contact with only a fraction of the waveguide surface. The thin-film waveguide is directly integrated with a planar fluidic device based on thermocapillary actuation. This system registers droplet position, sample con...