Droplet Jumping: Effects of Droplet Size, Surface Structure, Pinning, and Liquid Properties

Xiao Yan; Leicheng Zhang; Soumyadip Sett; Lezhou Feng; Chongyan Zhao; Zhiyong Huang; Hamed Vahabi; Arun K Kota; Feng Chen; Nenad Miljkovic

doi:10.1021/acsnano.8b06677

Droplet Jumping: Effects of Droplet Size, Surface Structure, Pinning, and Liquid Properties

ACS Nano. 2019 Feb 26;13(2):1309-1323. doi: 10.1021/acsnano.8b06677. Epub 2019 Jan 14.

Authors

Xiao Yan^{1

2}, Leicheng Zhang¹, Soumyadip Sett¹, Lezhou Feng¹, Chongyan Zhao², Zhiyong Huang², Hamed Vahabi³, Arun K Kota^{3

4

5}, Feng Chen², Nenad Miljkovic^{1

6

7

8}

Affiliations

¹ Department of Mechanical Science and Engineering , University of Illinois at Urbana-Champaign , Urbana , Illinois 61801 , United States.
² Institute of Nuclear and New Energy Technology , Tsinghua University , Beijing , 100084 , China.
³ Department of Mechanical Engineering , Colorado State University , Fort Collins , Colorado 80523 , United States.
⁴ School of Biomedical Engineering , Colorado State University , Fort Collins , Colorado 80523 , United States.
⁵ Department of Chemical Engineering , Colorado State University , Fort Collins , Colorado 80523 , United States.
⁶ Department of Electrical and Computer Engineering , University of Illinois at Urbana-Champaign , Urbana , Illinois 61801 , United States.
⁷ Frederick Seitz Materials Research Laboratory , University of Illinois at Urbana-Champaign , Urbana , Illinois 61801 , United States.
⁸ International Institute for Carbon Neutral Energy Research (WPI-I2CNER) , Kyushu University , 744 Moto-oka , Nishi-ku, Fukuoka , 819-0395 , Japan.

PMID: 30624899
DOI: 10.1021/acsnano.8b06677

Abstract

Coalescence-induced droplet jumping has the potential to enhance the efficiency of a plethora of applications. Although binary droplet jumping is quantitatively understood from energy and hydrodynamic perspectives, multiple aspects that affect jumping behavior, including droplet size mismatch, droplet-surface interaction, and condensate thermophysical properties, remain poorly understood. Here, we develop a visualization technique utilizing microdroplet dispensing to study droplet jumping dynamics on nanostructured superhydrophobic, hierarchical superhydrophobic, and hierarchical biphilic surfaces. We show that on the nanostructured superhydrophobic surface the jumping velocity follows inertial-capillary scaling with a dimensionless velocity of 0.26 and a jumping direction perpendicular to the substrate. A droplet mismatch phase diagram was developed showing that jumping is possible for droplet size mismatch up to 70%. On the hierarchical superhydrophobic surface, jumping behavior was dependent on the ratio between the droplet radius R_i and surface structure length scale L. For small droplets ( R_i ≤ 5 L), the jumping velocity was highly scattered, with a deviation of the jumping direction from the substrate normal as high as 80°. Surface structure length scale effects were shown to vanish for large droplets ( R_i > 5 L). On the hierarchical biphilic surface, similar but more significant scattering of the jumping velocity and direction was observed. Droplet-size-dependent surface adhesion and pinning-mediated droplet rotation were responsible for the reduced jumping velocity and scattered jumping direction. Furthermore, droplet jumping studies of liquids with surface tensions as low as 38 mN/m were performed, further confirming the validity of inertial-capillary scaling for varying condensate fluids. Our work not only demonstrates a powerful platform to study droplet-droplet and droplet-surface interactions but provides insights into the role of fluid-substrate coupling as well as condensate properties during droplet jumping.

Keywords: biphilic; coalescence-induced droplet jumping; directionality; jumping velocity; low-surface-tension liquid; superhydrophobic; visualization.

Publication types

Research Support, Non-U.S. Gov't
Research Support, U.S. Gov't, Non-P.H.S.