Microfluidic devices play an ever increasing role in nano- and biotechnologies. An emerging area of research in this technology-driven field is digital microfluidics which is based upon the micromanipulation of discrete droplets. Microfluidic processing is performed on unit-sized packets of fluid which are transported, stored, mixed, reacted, or analyzed in a discrete manner. An obvious challenge however is how to displace sessile droplets on a substrate. The video presented here illustrates a droplet actuation mechanism which relies on the mixing of two droplets with different properties. The resulting gradients in surface energies which exist for as long as the droplets have not fully mixed induce the motion of the droplet system. In the video, a 1 μl droplet of deionised water is first positioned on a hydrophilic stripe. This stripe which steers the droplet was obtained by plasma-treating a piece of PDMS shielded in some parts by glass coverslips. A 1 μl droplet of ethanol is then deposited with a micropipette in the vicinity of the prepositioned water droplet. The ethanol droplet spreads and when its advancing contact line meets the edge of the water droplet it induces the motion of the water droplet. Moving images were recorded with a high-speed camera (Phantom Miro V4, Vision Research) at a frame rate of 200 fps and post-processed using the Phantom Viewer software (V.9.2.675). The water droplet is displaced over 8.76 mm (two to three times its initial diameter) before coming to a halt. The average velocity of the water droplet is around 1.2 mm/s. Several feature are worthy of note in this video. Firstly, the image sequence gives the impression that the ethanol contact line is actually pushing the water droplet away. Secondly, the water droplet shape is no longer symmetrical during the motion, the advancing and receding contact angles taking different values. Thirdly, the droplet motion is not smooth, some form of stick-slip behaviour can be observed particularly towards the end of the propulsion. Finally, the motion only occurs for a limited time. We propose that the motion of the droplet is the result of complementary phenomena. Firstly, as the ethanol droplet meets the water droplet the surface tensions on either side of the droplet will be modified resulting in a net force on the water droplet. This force, analogous to the one described by Bico and Quéré in [1] for the motion of liquid slugs in closed capillaries, will exist for as long as the two droplets have not fully mixed. Secondly, as the two droplets mix, a surface tension gradient develops which induces Marangoni stresses. These contribute to the overall motion as discussed by Brochard, [2], and the present authors, [3]. This phenomenon is analogous to thermocapillary actuation but the surface tension gradient is induced by the mixing of the droplets instead of an applied temperature gradient. Finally, as the ethanol droplet spreads, the solid/air interface is replaced by a solid/ethanol interface which modifies the local substrate energy. This substrate energy gradient can also contribute to the droplet system motion, see [2,4,5]. Besides being pleasing to the eye, this video illustrates the simplicity and effectiveness of this droplet actuation mechanism.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the support of the Dumont d'Urville New Zealand / France Science & Technology Program.
[1] Bico, J.; Quéré, D. Self-propelling slugs in a tube. J. Fluid Mech. 2002, 467, 201.
[2] Brochard, F. Motions of droplets on solid surfaces induced by chemical or thermal gradients. Langmuir 1989, 5, 432.
[3] Sellier, M.; Nock, V.; Verdier, C. Self-propelling, coalescing droplets. Int. J. Multiphase Flow, in review.
[4] Chaudhury, M.K.; Whitesides, G.M. How to make water run uphill? Science 1992, 256, 1539.
[5] Domingues Dos Santos, F.; Ondarçhuhu, T. Free-running droplets. Phys. Rev. Lett. 1995, 75, 2972.
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