Energy Loss for Droplets Bouncing Off Superhydrophobic Surfaces

Calvin Thenarianto; Xue Qi Koh; Marcus Lin; Ville Jokinen; Dan Daniel

doi:10.1021/acs.langmuir.2c03449

Energy Loss for Droplets Bouncing Off Superhydrophobic Surfaces

Langmuir. 2023 Feb 28;39(8):3162-3167. doi: 10.1021/acs.langmuir.2c03449. Epub 2023 Feb 16.

Authors

Calvin Thenarianto¹, Xue Qi Koh¹, Marcus Lin², Ville Jokinen³, Dan Daniel^{1

4}

Affiliations

¹ Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), Singapore 138634.
² School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798.
³ Department of Chemistry and Materials Science, School of Chemical Engineering, Aalto University, Espoo 02150, Finland.
⁴ Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia.

PMID: 36795493
DOI: 10.1021/acs.langmuir.2c03449

Abstract

A water droplet can bounce off superhydrophobic surfaces multiple times before coming to a stop. The energy loss for such droplet rebounds can be quantified by the ratio of the rebound speed U_R and the initial impact speed U_I; i.e., its restitution coefficient e = U_R/U_I. Despite much work in this area, a mechanistic explanation for the energy loss for rebounding droplets is still lacking. Here, we measured e for submillimeter- and millimeter-sized droplets impacting two different superhydrophobic surfaces over a wide range of U_I (4-700 cm s^-1). We proposed simple scaling laws to explain the observed nonmonotonic dependence of e on U_I. In the limit of low U_I, energy loss is dominated by contact-line pinning and e is sensitive to the surface wetting properties, in particular to contact angle hysteresis Δ cos θ of the surface. In contrast, e is dominated by inertial-capillary effects and does not depend on Δ cos θ in the limit of high U_I.