Enhanced Water Nucleation and Growth Based on Microdroplet Mobility on Lubricant-Infused Surfaces

Abstract

Lubricant-infused surfaces (LISs) can promote stable dropwise condensation and improve heat transfer rates due to a low nucleation free-energy barrier and high droplet mobility. Recent studies showed that oil menisci surrounding condensate microdroplets form distinct oil-rich and oil-poor regions. These topographical differences in the oil surface cause water microdroplets to rigorously self-propel long distances, continuously redistributing the oil film and potentially refreshing the surface for re-nucleation. However, the dynamic interplay between oil film redistribution, microdroplet self-propulsion, and droplet nucleation and growth is not yet understood. Using high-speed microscopy, we reveal that during water condensation on LISs, the smallest visible droplets (diameter: ∼1 μm, qualitatively representing nucleation) predominantly emerge in oil-poor regions due to a lower nucleation free-energy barrier. Considering the significant heat transfer performance of microdroplets (<10 μm) and transient characteristic of microdroplet movement, we compare the apparent nucleation rate density and water collection rate for LISs with oils of different viscosities and a solid hydrophobic surface at a wide range of subcooling temperatures. Generally, the lowest lubricant viscosity leads to the highest nucleation rate density. We characterize the length and frequency of microdroplet movement and attribute the nucleation enhancement primarily to higher droplet mobility and surface refreshing frequency. Interestingly and unexpectedly, hydrophobic surfaces outperform high-viscosity LISs at high subcooling temperatures but are generally inferior to any of the tested LISs at low temperature differences. To explain the observed nonlinearity between LISs and the solid hydrophobic surface, we introduce two dominant regimes that influence the condensation efficiency: mobility-limited and coalescence-limited. We compare these regimes based on droplet growth rates and water collection rates on the different surfaces. Our findings advance the understanding of dynamic water–lubricant interactions and provide new design rationales for choosing surfaces for enhanced dropwise condensation and water collection efficiencies.

Bioinspired functional SLIPSs and wettability gradient surfaces and their synergistic cooperation and opportunities for enhanced condensate and fluid transport

“…Results revealed the larger microdroplet nucleation rate density in the oil-poor region, as shown in dashed red rectangle in Fig. 11(a), as a consequence of the reduced thermal conduction resistance imposed by the thinner oil film and the lower oil-air interfacial temperature preferential for nucleation [173,174] , which has also been demonstrated in the recent work of Sun et al [175] . Then droplets remained stationary growing through direct condensation at the nucleation sites and once they reached certain size the droplets suddenly coalesced with neighboring ones, as shown in Fig.…”

Marangoni-induced reversal of meniscus-climbing microdroplets

“…In Video 4 from the Supplemental Material, a ≈600-µm water droplet was left on the surface from previous condensation and coalescence events. At this relatively high vapor-substrate temperature difference (≈30°C), nucleation occurs not only in the thin film region in-between larger droplets, socalled oil-poor regions 11 , but also on the oil menisci themselves. The outward interfacial Marangoni flow can radially distribute these meniscus-nucleated droplets to regions further away from the central droplet, i.e., towards oil-poor regions.…”

“…Meniscus-mediated behaviors of droplets or particles can also easily be observed in our daily life, such as the cheerios effect, where cereals tend to aggregate and form rafts 5 , bubble clusters near the walls of sodafilled glasses, and oil drops floating on a bowl of water that migrate away from the solid wall 6 . Meniscusclimbing mechanisms have also been discovered and used for micro/nano-particle self-assembly 7 and enhanced water harvesting [8][9][10][11] . Manipulating small objects at liquid interfaces is furthermore becoming increasingly important in areas of microfluidics [12][13][14][15] , microrobotics 16 , and biological sensing and testing 17 .…”

Quantification of Nucleation Site Density as a Function of Surface Wettability on Smooth Surfaces

“…Liquid infused surfaces (LIS) have been shown to enhance droplet mobility in condensation applications, by dramatically reducing contact angle hysteresis, however the gradual loss of lubricant could make the effect short lived. [31,[36][37][38][39] Polymer infused porous surfaces also offer low contact angle hysteresis and tunability, as well as being more robust than LIS, however they are complex to fabricate and may be challenging to scale up. [37] Only recently, Cha et al have shown that a less complex hydrophilic surface with low contact angle hysteresis, can determine efficient dropwise condensation.…”