Coalescence-induced droplet jumping has the potential to enhance the efficiency of a plethora of applications. Although binary droplet jumping is quantitatively understood from energy and hydrodynamic perspectives, multiple aspects that affect jumping behavior, including droplet size mismatch, droplet−surface interaction, and condensate thermophysical properties, remain poorly understood. Here, we develop a visualization technique utilizing microdroplet dispensing to study droplet jumping dynamics on nanostructured superhydrophobic, hierarchical superhydrophobic, and hierarchical biphilic surfaces. We show that on the nanostructured superhydrophobic surface the jumping velocity follows inertial-capillary scaling with a dimensionless velocity of 0.26 and a jumping direction perpendicular to the substrate. A droplet mismatch phase diagram was developed showing that jumping is possible for droplet size mismatch up to 70%. On the hierarchical superhydrophobic surface, jumping behavior was dependent on the ratio between the droplet radius R i and surface structure length scale L. For small droplets (R i ≤ 5L), the jumping velocity was highly scattered, with a deviation of the jumping direction from the substrate normal as high as 80°. Surface structure length scale effects were shown to vanish for large droplets (R i > 5L). On the hierarchical biphilic surface, similar but more significant scattering of the jumping velocity and direction was observed. Droplet-size-dependent surface adhesion and pinning-mediated droplet rotation were responsible for the reduced jumping velocity and scattered jumping continued...

Superhydrophobicity has received significant attention over the past three decades owing to its significant potential in self-cleaning and anti-icing surfaces, drag reduction, energy harvesting devices, anti-bacterial coatings, and enhanced heat transfer applications. Superhydrophobicity can be obtained via the roughening of an intrinsically hydrophobic surface, the creation of a re-entrant geometry, or by the roughening of a hydrophilic surface followed by a conformal coating of a hydrophobic material. Intrinsically hydrophobic surfaces have poor thermophysical properties such as thermal conductivity, and thus are not suitable for heat transfer applications. Re-entrant geometries, although versatile in applications where droplets are deposited, break down during spatially random nucleation and flood the surface. Chemical functionalization of rough metallic substrates, although promising, is not utilized due to the poor durability of conformal hydrophobic coatings. Here we develop a radically different approach to achieve stable superhydrophobicity. By utilizing laser processing and thermal oxidation of copper (Cu) to create a high surface energy hierarchical copper oxide (CuO), followed by repeatable and passive atmospheric adsorption of hydrophobic volatile organic compounds (VOCs), we show that stable superhydrophobicity with apparent advancing contact angles ≈ 160° and contact angle hysteresis as low as ≈ 20° can be achieved. We exploit the structure length scale and structure geometry dependent VOC adsorption dynamics to rationally design CuO nanowires with enhanced superhydrophobicity. To gain an understanding of the VOC adsorption physics, we utilized X-Ray Photoelectron and Ion Mass Spectroscopy to identify the chemical species deposited on our surfaces in two distinct locations: Urbana, IL, USA and Beijing, China. To test the stability of the atmosphere-mediated superhydrophobic surfaces during heterogeneous nucleation, we used high-speed optical microscopy to demonstrate the occurrence of dropwise condensation and stable coalescenceinduced droplet jumping. Our work not only provides rational design guidelines for developing passively-durable superhydrophobic surfaces with excellent flooding-resistance and self-healing capability, but also sheds light on the key role played by the atmosphere in governing wetting.

With the recent advances in surface fabrication technologies, condensation heat transfer has seen a renaissance. Hydrophobic and superhydrophobic surfaces have all been employed as a means to enhance condensate shedding, enabling micrometric droplet departure length scales. One of the main bottlenecks for achieving higher condensation efficiencies is the difficulty of shedding sub-10 μm droplets due to the increasing role played by surface adhesion and viscous limitations at nanometric length scales. To enable ultraefficient droplet shedding, we demonstrate hierarchical condensation on rationally designed copper oxide microhill structures covered with nanoscale features that enable large (∼100 μm) condensate droplets on top of the microstructures to coexist with smaller (<1 μm) droplets beneath. We use high-speed optical microscopy and focal plane shift imaging to show that hierarchical condensation is capable of efficiently removing sub-10-μm condensate droplets via both coalescence and divergent-track-assisted droplet self-transport toward the large suspended Cassie−Baxter (CB) state droplets, which eventually shed via classical gravitational shedding and thereby avoid vapor side limitations encountered with droplet jumping. Interestingly, experimental growth rate analysis showed that the presence of large CB droplets accelerates individual underlying droplet growth by ∼21% when compared to identically sized droplets not residing beneath CB droplets. Furthermore, the steady droplet shedding mechanism shifted the droplet size distribution toward smaller sizes, with ∼70% of observable underlying droplets having radii of ≤5 μm compared to ∼30% for droplets growing without shading. To elucidate the overall heat transfer performance, an analytical model was developed to show hierarchical condensation has the potential to break the limits of minimum droplet departure size governed by finite surface adhesion and viscous effects through the tailoring of structure length scale, coalescence, and self-transport. More importantly, abrasive wear tests showed that hierarchical condensation has good durability against mechanical damage to the surface. Our study not only demonstrates the potential of hierarchical condensation as a means to break the limitations of droplet jumping, it develops rational design guidelines for micro/nanostructured surfaces to enable excellent heat transfer performance as well as extended durability.

Functional coatings that can achieve stable superhydrophobicity have the potential to significantly enhance a plethora of industrial applications ranging from building environmental control, phase change heat transfer, thermoelectric power generation, and hydrodynamic drag reduction. In order to create superhydrophobic surfaces, scientists have utilized a variety of surface structuring methods in combination with organosilane based alkyl and perfluorinated synthetic chemical coatings. Unfortunately, organosilane based alkyl and perfluorinated chemicals tend to be toxic, flammable, corrosive, difficult to dispose of, and damaging to the environment. Here, we develop two new methods to achieve superhydrophobicity using liquid phase deposition of cinnamic acid or myristic acid, both organic compounds derived from natural sources. By varying the liquid phase solution concentration, we develop deposition methods on scalable copper oxide microstructured surfaces capable of achieving apparent advancing contact angles as high as 154° and 165° for cinnamic and myristic acid, respectively, with low contact angle hysteresis (<15°). To demonstrate superhydrophobic performance, we utilize high speed optical microscopy to show stable coalescence induced droplet jumping during atmospheric water vapor condensation. This study presents a novel avenue for safer and more environmentally friendly fabrication of superhydrophobic surfaces for energy and water applications.

Particulate transport from surfaces governs a variety of phenomena including fungal spore dispersal, bioaerosol transmission, and self-cleaning. Here, we report a previously unidentified mechanism governing passive particulate removal from superhydrophobic surfaces, where a particle coalescing with a water droplet (∼10 to ∼100 μm) spontaneously launches. Compared to previously discovered coalescence-induced binary droplet jumping, the reported mechanism represents a more general capillary-inertial dominated transport mode coupled with particle/droplet properties and is typically mediated by rotation in addition to translation. Through wetting and momentum analyses, we show that transport physics depends on particle/droplet density, size, and wettability. The observed mechanism presents a simple and passive pathway to achieve self-cleaning on both artificial as well as biological materials as confirmed here with experiments conducted on butterfly wings, cicada wings, and clover leaves. Our findings provide insights into particle–droplet interaction and spontaneous particulate transport, which may facilitate the development of functional surfaces for medical, optical, thermal, and energy applications.

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Surface-tension-driven droplet coalescence has received significant attention due to its significant role in microfluidics, coalescence-induced droplet jumping, fluid mixing, and microscale heat and mass transfer. However, the study of internal flow characteristics of merging droplets remains a challenge due to visualization difficulty, limited droplet size control, poor droplet manipulation, and insufficient droplet front tracking. Here, in order to study droplet coalescence dynamics, a droplet dispensing and visualization system was developed. To control the size of droplets, monodispersed droplets with diameters of ≈ 40 μm were dispensed onto a superhydrophobic surface, enabling the target droplets to accumulate in volume and grow in radii. To track the internal flow front, an ethanol (20 wt %)-water mixture was used as the working fluid. Due to the unequal evaporation rate of water and ethanol, a density gradient was introduced at the liquid-gas interface of the droplets, resulting in a lower refractive index compared to that in the bulk liquid. The coalescence process of droplets with diameters of ≈ 232 μm and ≈ 1400 μm was imaged. We observed jet-like internal flow and vortex rings (“swimming jellyfish”) inside the merging droplet. It was shown that the jet-like flow underwent a significant deceleration while the vortex-ring experienced slow radial expansion. Our work not only presents a powerful platform capable of visualizing droplet coalescence hydrodynamics, but also provides insights into the internal flow dynamics during droplet coalescence.

Droplet coalescence has received significant attention due to its significant role in fluid mixing, microfluidics, coalescence-induced droplet jumping, and heat and mass transfer applications. Coalescence of droplets has been extensively investigated from the perspectives of hydrodynamics and energy transfer. However, the study of coalescence characteristics of size-mismatched droplets on superhydrophobic surfaces remains a challenge due to visualization difficulty, limited droplet size control, and poor droplet manipulation. Here, in order to study coalescence dynamics of droplets with arbitrary initial sizes, a droplet dispensing and visualization system was developed. To control the size of droplets, monodispersed droplets with radii of ≈20 μm were dispensed using a frequency-controlled piezoelectric pulse injector onto a superhydrophobic surface, enabling the target droplets to accumulate in volume and grow in radii. The coalescence process of droplets having radii of ≈270 and ≈780 μm was imaged at a magnification of ≈25X and capture rate of 13000 fps. Surprisingly instead of completely merging together, the size-mismatched droplets underwent partial coalescence with the development of an additional satellite droplet. Specifically, the smaller droplet gave 'birth' to a secondary satellite droplet upon coalescence with the larger primary droplet due to liquid-bridge pinch-off dynamics, after which the satellite droplet bounced off upon collision with the primary droplet due to the presence of an air cushion that blocked contact between the two droplets. Meanwhile, the primary droplet continued to oscillate while the bouncing satellite droplet returned to the surface and eventually bounced off (moving direction is identified with arrows). Our work not only presents a powerful platform capable of both controlling and visualizing droplet coalescence hydrodynamics, but also provides insights into the flow hydrodynamics of droplets undergoing partial coalescence.