Cylindrical chains of water drops condensing on microstructured lubricant-infused surfaces

Kajiya, Tadashi; Wooh, Sanghyuk; Lee, Yunchan; Char, Kookheon; Vollmer, Doris; Butt, Hans‐Jürgen

doi:10.1039/c6sm01883a

Cited by 14 publications

(17 citation statements)

References 28 publications

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“…16,17 The meniscus and cloak lead to increased viscous dissipation and lower droplet sliding velocities. [18][19][20][21][22][23][24] On the other hand, overlapping menisci can also induce a net attractive force between droplets, [25][26][27][28] potentially enhancing gravity-induced droplet sweeping. Frequent droplet sweeping is essential to remove condensate, to renew nucleation sites, and to maintain high heat transfer rates during condensation.…”

Section: Introductionmentioning

confidence: 99%

Microdroplet self-propulsion during dropwise condensation on lubricant-infused surfaces

Sun

Weisensee

2019

Soft Matter

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Water vapor condensation is common in nature and widely used in industrial applications, including water harvesting, power generation, and desalination. As compared to traditional filmwise condensation, dropwise condensation on lubricant-infused surfaces (LIS) can lead to an order-of-magnitude increase in heat transfer rates. Small droplets (D ≤ 100 μm) account for nearly 85 % of the total heat transfer and droplet sweeping plays a crucial role in clearing nucleation sites, allowing for frequent re-nucleation. Here, we focus on the dynamic interplay of microdroplets with the thin lubricant film during water vapor condensation on LIS. Coupling high-speed imaging, optical microscopy, and interferometry, we show that the initially uniform lubricant film re-distributes during condensation. Governed by lubricant height gradients, microdroplets as small as 2 μm in diameter undergo rigorous and gravity-independent selfpropulsion, travelling distances multiples of their diameters at velocities up to 1100 µm/s. Although macroscopically the movement appears to be random, we show that on a microscopic level capillary attraction due to asymmetrical lubricant menisci causes this gravity-independent droplet motion. Based on a lateral force balance analysis, we quantitatively find that the sliding velocity initially increases during movement, but decreases sharply at shorter inter-droplet spacing. The maximum sliding velocity is inversely proportional to the oil viscosity and is strongly dependent of the droplet size, which is in excellent agreement with the experimental observations. This novel and non-traditional droplet movement is expected to significantly enhance the sweeping efficiency during dropwise condensation, leading to higher nucleation and heat transfer rates.

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Section: Introductionmentioning

confidence: 99%

Microdroplet self-propulsion during dropwise condensation on lubricant-infused surfaces

Sun

Weisensee

2019

Soft Matter

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“…These image sequences clearly reveal that coalescence can be suppressed by application of an electric field. When the electric field is switched off, rapid coalescence is observed, presumably promoted by the short-range capillary attraction between the droplets 40 – 42 . With electric field, however, not a single coalescence event is observed even for time intervals much larger than that covered by the image sequence of Fig.…”

Section: Resultsmentioning

confidence: 99%

“…First, owing to the low contact-angle hysteresis of droplets on LIS, their motion along the surface gives reliable and reproducible information about the forces acting on them. Second, suppressing the coalescence of droplets on LIS is especially challenging, since the oil wetting ridge surrounding the droplets 37 – 39 induces short-range attractive forces that promote coalescence 40 – 42 . Apart from that, in the past few years controlled droplet manipulation on LIS was demonstrated 40 , 43 – 45 , making such systems open droplet microfluidic platforms in their own right.…”

Section: Introductionmentioning

confidence: 99%

Manipulation and control of droplets on surfaces in a homogeneous electric field

2022

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A method to manipulate and control droplets on a surface is presented. The method is based on inducing electric dipoles inside the droplets using a homogeneous external electric field. It is shown that the repulsive dipole force efficiently suppresses the coalescence of droplets moving on a liquid-infused surface (LIS). Using a combination of experiments, numerical computations and semi-analytical models, the dependence of the repulsion force on the droplet volumes, the distance between the droplets and the electric field strength is revealed. The method allows to suppress coalescence in complex multi-droplet flows and is real-time adaptive. When the electric field strength exceeds a critical value, tip streaming from the droplets sets in. Based on that, it becomes possible to withdraw minute samples from an array of droplets in a parallel process.

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“…In the case of a cluster of microdroplets, the radially inward capillary attraction balanced by the outward push due to the capillary deformation of droplets brings an equilibrium. Such capillary interactions are reported, for example, in the cases of mimicking the cellcell adhesion pressure using functionalized emulsion droplets 34 , capillary orbiting of silicone oil droplets dispersed on liquid nitrogen surface 35 , aqueous droplets placed closely on oil-coated surfaces [36][37][38] , droplets sliding on gels 39 , and colloidal particles suspended at water-air interface 40 .…”

Section: Principlementioning

confidence: 99%

Evaporating microfluidic droplets: A tool to study pathogen viability in bioaerosols

Gopu,

Agrawal,

Mukherjee

et al. 2021

Preprint

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Pathogens in droplets on fomites and aerosols go through extreme physiochemical conditions, such as confinement and osmotic stress, due to evaporation. Still, these droplets are the predominant transmission routes of many contagious diseases. The biggest challenge for studying the survival mechanisms of pathogens in extreme conditions is closely observing them, especially in aerosols, due to the small droplet sizes, movements, and fast evaporative dynamics. To mimic evaporating aerosols and microdroplets, we employ microfluidic emulsion droplets. The presence of oil forms a microdroplet cluster with spatially gradient evaporation rates, which helps studying the impact of evaporation and its rate. With Escherichia coli, we show that the viability is adversely affected by the evaporation and its rate. Our method can mimic bioaerosol evaporation with controlled number of cells inside. In general, the droplets can act as microreactors with varying kinetics induced by different evaporation rates.

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Cylindrical chains of water drops condensing on microstructured lubricant-infused surfaces

Cited by 14 publications

References 28 publications

Microdroplet self-propulsion during dropwise condensation on lubricant-infused surfaces

Microdroplet self-propulsion during dropwise condensation on lubricant-infused surfaces

Manipulation and control of droplets on surfaces in a homogeneous electric field

Evaporating microfluidic droplets: A tool to study pathogen viability in bioaerosols

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