Directional Movement of Droplets in Grooves: Suspended or Immersed?

Abstract

The behavior of droplets trapped in geometric structures is essential to droplet manipulation applications such as for droplet transport. Here we show that directional droplet movement can be realized by a V-shaped groove with the movement direction controlled by adjusting the surface wettability of the groove inner wall and the cross sectional angle of the groove. Experiments and analyses show that a droplet in a superhydrophobic groove translates from the immersed state to the suspended state as the cross sectional angle of the groove decreases and the suspended droplet departs from the groove bottom as the droplet volume increases. We also demonstrate that this simple grooved structure can be used to separate a water-oil mixture and generate droplets with the desired sizes. The structural effect actuated droplet movements provide a controllable droplet transport method which can be used in a wide range of droplet manipulation applications.

Effects of wettability on droplet movement in a V-shaped groove

“…In summary, when the inner walls of the V-shaped groove are hydrophobic or superhydrophobic, the upper meniscus of the suspended droplet is the leading meniscus while V drop changes. This expectation matches the experimental results from a previous study 6 well. Present study also demonstrated this expectation experimentally.…”

“…The radius of the curvature of the upper meniscus is expressed as Therefore, when a droplet is gently placed on the surface, the upper and lower menisci are convex, because θ s > 90°. In addition, the droplet is almost spherical to minimize the surface energy 6 . Under these conditions, θ u = θ d .…”

“…Droplet manipulation to control the dynamic behavior or wetting modes of droplets is an essential technique for various applications such as increasing the efficiency of condensation heat transfer 1 – 4 , water–oil separation 5 , 6 , fabricating microlens arrays 7 , and energy harvesting by electrically-charged jumping droplets 8 , 9 . Initially, the droplet manipulation was achieved by chemical or thermal gradient methods 10 – 14 .…”

“…Afterwards, natural phenomena such as directional movements of droplets on a lotus leaf 15 , wetted spider silk 16 and cacti 17 have demonstrated that the droplet manipulation can be achieved by geometric structures. Since then, many types of structured surfaces 5 , 6 , 18 – 23 have been suggested for the droplet manipulation.…”

Unidirectional transport of water nanodroplets entrapped inside a nonparallel smooth surface: a molecular dynamics simulation study

“…Such phenomena can be helpful for water harvesting induced by condensation, 19 to transport liquid droplets in micro/nano uidic systems 20 and to produce a liquid drop of a desired size. 21 The spontaneous propulsion of liquid droplets bridged between NPSs is usually observed in hydrophilic surfaces but the underlying physical principal of such a phenomenon is still not clear. 22 In one of the studies it was observed that the dihedral angle of two NPSs plays a crucial role in deciding the stability of the liquid bridge.…”