Breaking Droplet Jumping Energy Conversion Limits with Superhydrophobic Microgrooves

Abstract

Coalescence-induced droplet jumping has the potential to enhance the performance of a variety of applications including condensation heat transfer, surface self-cleaning, anti-icing, and defrosting to name a few. Here, we study droplet jumping on hierarchical microgrooved and nanostructured smooth superhydrophobic surfaces. We show that the confined microgroove structures play a key role in tailoring droplet coalescence hydrodynamics, which in turn affects the droplet jumping velocity and energy conversion efficiency. We observed self-jumping of individual deformed droplets within microgrooves having maximum surface-to-kinetic energy conversion efficiency of 8%. Furthermore, various coalescence-induced jumping modes were observed on the hierarchical microgrooved superhydrophobic surface. The microgroove structure enabled high droplet jumping velocity (≈0.74U) and energy conversion efficiency (≈46%) by enabling the coalescence of deformed droplets in microgrooves with undeformed droplets on adjacent plateaus. The jumping velocity and energy conversion efficiency enhancements are 1.93× and 6.67× higher than traditional coalescence-induced droplet jumping on smooth superhydrophobic surfaces. This work not only demonstrates high droplet jumping velocity and energy conversion efficiency but also demonstrates the key role played by macroscale structures on coalescence hydrodynamics and elucidates a method to further control droplet jumping physics for a plethora of applications.

Laplace Pressure Driven Single-Droplet Jumping on Structured Surfaces

“…42 Surface-structure-induced Laplace pressure contrast within droplets has been identified as a potential driving force for droplet wetting state transition, 46,47 self-transport along diverging channels, 12,28,30,48,49 and self-removal from micropores. 45,50 However, the role of surface structures on Laplace pressure enabled droplet transport remains unexplored and droplet transport performance remains to be enhanced. In addition to the passive methods described above, active approaches to promote droplet jumping via external forces such as electric forces 16,51 require additional energy input.…”

“…Among them, increasing the surface nonwettability through surface nanostructure and chemistry has reached the physics-based limitations. ,, Even for ideal superhydrophobic surfaces having no work of adhesion or pinning, the jumping velocity of the coalesced droplet pair is still limited by coalescence hydrodynamics. Although tailoring of the droplet–surface impact hydrodynamics via macroscale surface structures can increase the energy conversion efficiency to 40% , and alter directionality, , the need for exquisite droplet placement , makes this approach untenable for phenomena governed by the spatially random nucleation of droplets . Surface-structure-induced Laplace pressure contrast within droplets has been identified as a potential driving force for droplet wetting state transition, , self-transport along diverging channels, ,,,, and self-removal from micropores. , However, the role of surface structures on Laplace pressure enabled droplet transport remains unexplored and droplet transport performance remains to be enhanced.…”

Numerical simulation of the coalescence-induced polymeric droplet jumping on superhydrophobic surfaces

“…Coalescence-induced jumping has been reported on a variety of natural repellent surfaces such as cicada, lacewings [30] and gecko skin [29], and can be exploited in a variety of applications such as anti-icing [36] and self-cleaning surfaces [29,30], and to control heat transfer [10]. Several researchers have studied the different aspects of the coalescence-induced droplet jumping numerically and experimentally, including the basic mechanism of the two equal-sized drop self-propelled jumping [19], and the effects of droplet size mismatch [28,27], droplet initial velocity [14,18], surface topology [26,24,34,20,23], surrounding gas properties [11,32,33], and surface wettability [6]. A few main results are outlined in the following.…”

Improving heat and mass transfer rates through continuous drop-wise condensation

“…This improvement is highly dependant on the shedding frequency and size of droplets on surfaces 21 . Droplets shedding has been achieved primarily by gravity assistance 22 – 24 , droplet jumping 14 , 25 – 27 , drag force 28 – 32 , or by capillary driven movement 33 , 34 . It has been widely accepted that droplets of diameters below 20 micron contribute about 80% of the total heat transfer to the surface 35 .…”