Directional Passive Transport of Microdroplets in Oil-Infused Diverging Channels for Effective Condensate Removal

Abstract

Condensation widely exists in nature and industry, and its performance heavily relies on the efficiency of condensate removal. Recent advances in micro-/nanoscale surface engineering enable condensing droplet removal from solid surfaces without extra energy cost, but it is still challenging to achieve passive transport of microdroplets over long distances along horizontal surfaces. The mobility of these condensate droplets can be enhanced by lubricant oil infusion on flat surfaces and frequent coalescence, which lead to fast growth but random motion of droplets. In this work, we propose a novel design of diverging microchannels with oil-infused surfaces to achieve controllable, long-distance, and directional transport of condensing droplets on horizontal surfaces. This idea is experimentally demonstrated with diverging copper and silicon microchannels with nanoengineered surfaces. Along these hierarchical surface structures, microdroplets condense on the top channel wall and submerge into microchannels owing to the capillary pressure gradient in infusing oil. Confined by the microchannel walls, the submerged droplets deform and maintain the back-front curvature difference, which enables the motion of droplets along the channel diverging direction. Subsequent droplet coalescences inside the channel further enhance this directional transport. Moreover, fast-moving deformed droplets transfer their momentum to downstream spherical droplets through the infusing oil. As a result, simultaneous passive transport of multiple droplets (20-400 μm) is achieved over long distances (beyond 7 mm). On these oil-infused surfaces, satellite microdroplets can further nucleate and grow on an oil-cloaked droplet, demonstrating an enlarged surface area for condensation. Our findings on passive condensate removal offer great opportunities in condensation enhancement, self-cleaning, and other applications requiring directional droplet transport along horizontal surfaces.

Self-Sustained Cascading Coalescence in Surface Condensation

“…9 Inspired by natural hydrophobic surfaces capable of passive removal of droplets, multiple artificial surfaces have been proposed, which achieve droplet shedding through gravity by using hydrophobic substrates 9,10 or by causing extreme sliding through lubricant-infused texturing. 11−14 Further enhancement in dropwise condensation has been achieved through gravity-independent droplet removal by utilizing droplet coalescence-induced jumping on superhydrophobic surfaces 15−18 or by causing directional droplet movement with capillarity gradients. 19,20…”

Laplace Pressure Driven Single-Droplet Jumping on Structured Surfaces

“…In the industrial world, rapid shedding of droplets from solid surfaces has become the primary bottleneck hindering the enhancement of a plethora of applications including self-cleaning, antifogging, antifrosting, − water harvesting, and condensation heat transfer. − In an effort to enhance droplet shedding, various approaches have been proposed to realize passive or active droplet transport enabled by external electric, − vapor cushion, and capillary and gravitational forces. Among them, droplet self-transport induced by surface chemistry and/or structural properties including wettability gradients, ,, charge density gradients, structure variations, − and diverging tracks ,− have received increasing attention due to their passive nature. However, chemistry/structure-gradient-based droplet self-transport gives rise to low transport velocities with limited travel distance. ,, More importantly, the scalable fabrication of surfaces with chemistry or structural variations remains challenging ,, and represents the main barrier to entry for real-life applications of functional surfaces …”

“…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. In addition to the passive methods described above, active approaches to promote droplet jumping via external forces such as electric forces , require additional energy input.…”

Biomimetic on-chip filtration enabled by direct micro-3D printing on membrane

“…As an alternative, chemical-free anti-fouling/clogging strategy becomes highly attractive. Surface patterning, creating topological structures on membrane surfaces, can manipulate the local hydrodynamics and the corresponding foulant-surface interaction 25 – 30 . With properly designed surface structures, the flow field near the membrane surface can be controlled to inhibit the deposition and accumulation of foulants particularly micro-sized foulant particles or droplets.…”