Thermocapillary Actuation

Chen JZ, Darhuber AA, Troian SM, Wagner S (2004) Capacitive sensing of droplets for microfluidic devices based on thermocapillary actuation. Lab Chip 4 473 80... 

Droplet Microreactor, Fig. 2 Droplet microreactor based on thermocapillary actuation, (a) Without a temperature gradient, the droplet is at equilibrium and does not move, (b) If heater 1 is on, the induced temperature gradient propels the droplet owing to the difference in surface stress between the sides of the droplet, (c) If the droplet moves out of the temperature field of heater 1, heater 2 is activated to propel the droplet further... 

Darhuber AA, Valentino JP, Troian SM, Wagner S (2003) Thermocapillary actuation of droplets on chemically patterned surfaces by programmable microheater arrays. J Microelectromech Syst 12 873-879... 

A.A. Darhuber, J.M. Davis, S.M. Troian, W.W. Reisner, Thermocapillary actuation of liquid flow on chemically patterned surfaces. Physics of Fluids, 2003, 15, 1295-1304. 

The one-dimensional droplet transport model, presented as above, is somewhat generic in nature and can take special forms depending on the specific modes of droplet motion actuation. For example, one may consider the thermocapillary-driven droplet motion in a cylindrical capillary, in which the surface tension varies as a function of the local temperature. For small temperature variations, this dependence is approximately linear and can be described as... 

There are two basic platforms for droplet microreactors the planar platform and the in-channel continuous platform. In a planar platform, the droplet can move freely on a planar surface, while the motion of the droplet in an in-channel continuous platform is restricted by microchannels. The actuation of droplets in a planar platform is based on nonmechanical concepts such as electrowettmg, thermocapillary forces, and magnetic forces. Most in-channel... 

ElectrocapUlary phenomenon refers to the modification of the interfacial tension by the presence of electrical charges. The first comprehensive investigations on electrocapillary phenomena were performed by Lippman, way back in 1875 [1]. In Lippman s experimental apparatus, the interfacial tension modulation due to electrical effects was observed through a capillary rise phenomenon and hence was later termed as electrocapillarity. A decisive advantage of electrocapillary actuation, in comparison to its thermal counterpart (i.e., the thermocapUlary effect, in which surface tension differentials are created by imposed temperature gradients), is the speed with which electrical potentials can be applied and regulated, with possible characteristic timescales of even less than a few milliseconds. Further, electrocapdlary-based microactuators consume much less power, as compared to the typical thermocapillary microdevices. 

Surface-directed microfluidic devices are inherently simple. Consequently, flow modulation is a concern. Methods have been employed to gain more control over flow such as mechanical actuation and thermocapillary induction. Moreover, material coatings provide the potential for the development of flow rate impediment and passive gate introduction through contraphilism or light mediation. 

Transport of droplets by thermal capillarity is an actuation concept utilizing thermocapillary forces. A temperature gradient across a liquid droplet leads to a difference in surface tensions at both ends of the droplet. The surface tension difference drives the liquid droplet toward the cooler place. 

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