Liquid metal droplets bouncing higher on thicker water layer

Yuhang Dai; Minfei Li; Bingqiang Ji; Xiong Wang; Siyan Yang; Peng Yu; Steven Wang; Chonglei Hao; Zuankai Wang

doi:10.1038/s41467-023-39348-x

Liquid metal droplets bouncing higher on thicker water layer

Nat Commun. 2023 Jun 14;14(1):3532. doi: 10.1038/s41467-023-39348-x.

Authors

Yuhang Dai^#^{1

2}, Minfei Li^#¹, Bingqiang Ji^#¹, Xiong Wang¹, Siyan Yang¹, Peng Yu², Steven Wang³, Chonglei Hao⁴, Zuankai Wang^{5

6}

Affiliations

¹ Department of Mechanical Engineering, City University of Hong Kong, Hong Kong, 999077, China.
² Department of Mechanical and Aerospace Engineering, Southern University of Science and Technology, Shenzhen, 518055, China.
³ Department of Mechanical Engineering, City University of Hong Kong, Hong Kong, 999077, China. steven.wang@cityu.edu.hk.
⁴ School of Mechanical Engineering and Automation, Harbin Institute of Technology, Shenzhen, 518055, China. haoc@hit.edu.cn.
⁵ Department of Mechanical Engineering, City University of Hong Kong, Hong Kong, 999077, China. zk.wang@polyu.edu.hk.
⁶ Department of Mechanical Engineering, Hong Kong Polytechnic University, Hong Kong, 999077, China. zk.wang@polyu.edu.hk.

^# Contributed equally.

Abstract

Liquid metal (LM) has gained increasing attention for a wide range of applications, such as flexible electronics, soft robots, and chip cooling devices, owing to its low melting temperature, good flexibility, and high electrical and thermal conductivity. In ambient conditions, LM is susceptible to the coverage of a thin oxide layer, resulting in unwanted adhesion with underlying substrates that undercuts its originally high mobility. Here, we discover an unusual phenomenon characterized by the complete rebound of LM droplets from the water layer with negligible adhesion. More counterintuitively, the restitution coefficient, defined as the ratio between the droplet velocities after and before impact, increases with water layer thickness. We reveal that the complete rebound of LM droplets originates from the trapping of a thinly low-viscosity water lubrication film that prevents droplet-solid contact with low viscous dissipation, and the restitution coefficient is modulated by the negative capillary pressure in the lubrication film as a result of the spontaneous spreading of water on the LM droplet. Our findings advance the fundamental understanding of complex fluids' droplet dynamics and provide insights for fluid control.