Droplet spontaneous motion

A surface whose physicochemical properties gradually change as a function of the position is called a gradient surface. While the gradient surface has potential applications as a substrate for combinatorial studies [29-31], the gradient of surface wetting is of particular interest because spontaneous motions of a droplet are induced on the surface [32]. 

Manipulation of a droplet on a solid surface is of growing interest because it is a key technology to construct lab-on-a-chip systems. The imbalance of surface tensions is known to cause spontaneous motion of a droplet on the surface, as mentioned above. The wetting gradient causing liquid motion has been prepared by chemical [32], thermal [37], electrochemical [3] and photochemical [38-40] methods. 

Figure 16.4 Spontaneous motion of a droplet on wetting gradients.

M 15] [P 15] In order to demonstrate the basic functionality of the dielectrophoretic fluid processor, some complex droplet motions were made (see Figure 1.43) [99], As an example, one 0.18 nl and two 0.065 nl droplets were guided over an 8 x 8 array (30 pm electrodes) and one after the other combined with a 0.38 nl droplet. Spontaneous fusion was observed. In this way, a 0.69 nl droplet resulted, containing all the initial volumes. This droplet was finally moved over the array. A speed of >1.8 mm s 1 was achieved. 

Nagai, K., Sumino, Y., Kitahata, H., Yoshikawa, K. Mode selection in the spontaneous motion of an alcohol droplet. Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 71 (6 Pt 2),... 

With respect to flocculation of nano-emulsion droplets, it is not clear whether such droplets can adhere and form a thin flat film, as do large drops. On the one hand, because of their small size, the curvature is very high and the Laplace pressiue opposes deformation. On the other hand, thermal agitation of small droplets (Brownian motion) can increase collisions and enhance deformation [70]. Anyway, flocculation is achieved spontaneously if the profile of the interaction energy as a function of the separation distance has a minimum deep enough to overcome the thermal energy of the droplets. 

Figure 1.43 Multiple droplet motion and fusion on an 8 x 8 array fluid processor with 30 pm electrodes, (a) Four droplets of varying size, three on one line and one separate large droplet (b) spontaneous fusion of the middle droplet on the line with the large droplet (c) spontaneous fusion of the upper droplet on the line with the large droplet ...

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