Controlling the Jumping Angle of Coalescing Droplets Using Surface Structures

Abstract

The jumping direction is an essential characteristic of jumping droplets, but it is poorly understood and uncontrollable at present. In this work, we present a method to control the jumping direction by surface structures, where the jumping direction is controlled by changing the inclination angle of the structure. The underlying mechanism is analyzed experimentally, with numerical simulations, and using a theoretical model developed to relate the jumping direction and the inclination angle for a few cases with a specific distribution. Because random droplet distributions are more common on actual condensation surfaces, a more comprehensive prediction model was developed based on a convolution neural network (CNN) to predict the jumping direction for more general cases. The input to the CNN is an image of droplets with various distribution features, which are detected by the neural network and used to predict the jumping angle. SHapley Additive exPlanations methods were then used to analyze the feature importance and to give human-understandable insights from the prediction model. This work offers avenues for improving cooling rates, anti-icing/freezing characteristics, and self-cleaning attributes and contributes to a better understanding of the jumping direction.

Condensation droplet sieve

“…3c shows the relationship between the energy-conversion efficiency and the coalescence mismatch after two-droplet coalescence (Supplementary Movie 4 ). The results are calculated using the customized VOF solver JumpingFOAM based on OpenFOAM, which is particularly designed to handle jumping droplet simulations 52 , 53 (Supplementary Discussion 7 ). The initial configurations of a single droplet of being in touch with the lattice bottom or being suspended on the top of the lattice, are predicted theoretically.…”

Enhancement and Guidance of Coalescence-Induced Jumping of Droplets on Superhydrophobic Surfaces with a U-Groove

“…Wang et al reported a millimetric triangular prism macrotexture and achieved a maximum droplet jumping velocity V j * = 0.53, with an energy conversion efficiency η ≈ 22.49% . In addition, rectangular ridge structures, curved surfaces, hydrophobic fibers, strings, and V-shaped structures , have been used to obtain a higher jumping velocity and break the velocity limit. Furthermore, Yuan et al used the impact between the liquid bridge and the bottom, flank side of the L-shaped macrotexture.…”

High-Efficiency Directional Ejection of Coalesced Drops on a Circular Groove

“…28 Yuan et al demonstrated that the jumping direction could be controlled by changing the inclination angle of the structure. 29 Lately, meticulously designed surface structures, such as L-shaped macrostructure, 30 asymmetric grooves, 31 and inclined strings, 23 have been applied in adjusting the jumping direction with energy conversion efficiency of around 30%. Recently, Liu et al reported tunable droplet jumping with energy efficiency up to 46%, but it requires the complex fabrication of three-dimensional sub-millimeter features of a U groove on a semicircular plate.…”

“…Specifically, jumping droplets with in-plane velocity have a sweeping effect on surrounding droplets by absorbing more minor droplets after departure, which cleans out more surface area . Yuan et al demonstrated that the jumping direction could be controlled by changing the inclination angle of the structure . Lately, meticulously designed surface structures, such as L-shaped macrostructure, asymmetric grooves, and inclined strings, have been applied in adjusting the jumping direction with energy conversion efficiency of around 30%.…”