We report a self-propelled jumping phenomenon for coalescing drops on superhydrophobic surfaces. The spontaneous motion is powered by surface energy released upon coalescence.1,2
On a horizontal, chilled superhydrophobic surface with an apparent contact angle close to , coalescing drops of water condensate spontaneously jump out of plane, against gravity. In the capillary-inertial regime, the velocity scales as , where is the surface tension, is the density, and is the drop radius before coalescence. Therefore, smaller drops jump with a higher velocity (Fig. 1), as long as the drop diameter is above where viscous effects are not dominating.2
![FIG. 1. On a superhydrophobic surface, water condensate spontaneously jump upon drop coalescence. In the capillary-inertial regime, smaller drops jump higher: coalescing drops are 200 and 270μm in diameter in (a), and 80 and 90μm in (b) (enhanced online). [URL: http://dx.doi.org/10.1063/1.3483222.1]](/view-large/figure/76914496/091110_1_f1.jpg)
On a superhydrophobic surface, water condensate spontaneously jump upon drop coalescence. In the capillary-inertial regime, smaller drops jump higher: coalescing drops are 200 and in diameter in (a), and 80 and in (b) (enhanced online). [URL: http://dx.doi.org/10.1063/1.3483222.1]
On a superhydrophobic surface, water condensate spontaneously jump upon drop coalescence. In the capillary-inertial regime, smaller drops jump higher: coalescing drops are 200 and in diameter in (a), and 80 and in (b) (enhanced online). [URL: http://dx.doi.org/10.1063/1.3483222.1]
On a heated Leidenfrost surface with liquid drops floating on a vapor layer which resembles superhydrophobicity, the mechanism of the out-of-plane directionality was revealed by releasing two drops from opposing synchronized gates to induce coalescence. The jumping motion resulted from the evolving liquid bridge of the coalescing drops impinging against the substrate [Fig. 2(a)]; the liquid bridge initiated well above the surface because of the high contact angle of the drops. As a confirmation of the impingement mechanism, when bouncing drops coalesced away from the substrate, no appreciable jumping occurred [Fig. 2(b)].
On a Leidenfrost surface, coalescing water drops spontaneously jump when the liquid bridge impacts the substrate (a). This mechanism is confirmed in (b) where coalescence away from the substrate does not lead to jumping.
On a Leidenfrost surface, coalescing water drops spontaneously jump when the liquid bridge impacts the substrate (a). This mechanism is confirmed in (b) where coalescence away from the substrate does not lead to jumping.
This work was supported by the National Science Foundation (Contract Nos. CBET-08-40370 and CBET-08-46705) and the Defense Advanced Research Projects Agency (Contract No. N66001-08-C-2009).
