Shuhuai Yao

Recurrent Filmwise and Dropwise Condensation on a Beetle Mimetic Surface

Vapor condensation plays a key role in a wide range of industrial applications including power generation, thermal management, water harvesting and desalination. Fast droplet nucleation and efficient droplet departure as well as low interfacial thermal resistance are important factors that determine the thermal performances of condensation; however, these properties have conflicting requirements on the structural roughness and surface chemistry of the condensing surface or condensation modes (e.g., filmwise vs dropwise). Despite intensive efforts over the past few decades, almost all studies have focused on the dropwise condensation enabled by superhydrophobic surfaces. In this work, we report the development of a bioinspired hybrid surface with high wetting contrast that allows for seamless integration of filmwise and dropwise condensation modes. We show that the synergistic cooperation in the observed recurrent condensation modes leads to improvements in all aspects of heat transfer properties including droplet nucleation density, growth rate, and self-removal, as well as overall heat transfer coefficient. Moreover, we propose an analytical model to optimize the surface morphological features for dramatic heat transfer enhancement.

Activating the Microscale Edge Effect in a Hierarchical Surface for Frosting Suppression and Defrosting Promotion

Despite extensive progress, current icephobic materials are limited by the breakdown of their icephobicity in the condensation frosting environment. In particular, the frost formation over the entire surface is inevitable as a result of undesired inter-droplet freezing wave propagation initiated by the sample edges. Moreover, the frost formation directly results in an increased frost adhesion, posing severe challenges for the subsequent defrosting process. Here, we report a hierarchical surface which allows for interdroplet freezing wave propagation suppression and efficient frost removal. The enhanced performances are mainly owing to the activation of the microscale edge effect in the hierarchical surface, which increases the energy barrier for ice bridging as well as engendering the liquid lubrication during the defrosting process. We believe the concept of harnessing the surface morphology to achieve superior performances in two opposite phase transition processes might shed new light on the development of novel materials for various applications.

Why Condensate Drops Can Spontaneously Move Away on Some Superhydrophobic Surfaces but Not on Others

The coalesce-induced condensate drop motion on some superhydrophobic surfaces (SHSs) has attracted increasing attention because of its wide potential applications. However, microscopic mechanism of spontaneous motion has not been discussed thoroughly. In this study, we fabricated two types of superhydrophobic copper surfaces with sisal-like nanoribbon structures and defoliation-like nanosheet structures by different wet chemical oxidation process and followed by same fluorization treatment. With lotus leaf and butterfly wing as control samples, the spontaneous motion phenomenon of condensate drops on these four kinds of SHSs was investigated by using optical microscope under ambient conditions. The results showed that among all four types of SHSs, only superhydrophobic copper surfaces with sisal-like nanoribbon structures showed obvious spontaneous motion of condensate drops, especially when the relative humidity was higher. The microscopic mechanism of spontaneous motion was discussed in relation to the states of condensate drops on different nanostructures. It shows that the instantaneous Cassie state of condensed droplets prior to coalescence plays a key role in determining whether the coalesced drop departs, whereas only SHS possessing nanostructures with small enough Wenzel roughness parameter r (at least <2.1) and nanogaps forming high enough Laplace pressure favors the formation of the instantaneous Cassie state by completing the Wenzel-Cassie transition.

Mechanism of Delayed Frost Growth on Superhydrophobic Surfaces with Jumping Condensates: More Than Interdrop Freezing

Delayed frost growth on superhydrophobic surfaces (SHSs) with jumping condensates has been found by many researchers recently. However, the mechanism of this phenomenon has not been elucidated clearly. In this study, copper SHSs with or without jumping condensates were selected as the substrates for observing condensation icing at a relative humidity (RH) of 60%. The results showed that only SHS with jumping condensates showed delayed condensation icing. Moreover, when such SHSs were placed upward and the surface temperature was held at -10 °C, some discrete frozen drops first appeared on the SHSs. The following icing mainly occurred on these discrete global crystals and then expanded around them until covering the entire surface. Little macroscopic interdrop freezing phenomenon was found. The growth of the frost front is mainly dominated by jumping freezing (the condensed droplets jumped onto the ice crystals and were frozen) or direct vapor-ice deposition. Using microscopy, we found interdrop freezing occurred, in addition to the two mechanisms mentioned above. By placing the SHS downward at -10 °C and intentionally introducing or eliminating tiny dusts, we confirmed that there were no superhydrophobic defects on our SHSs. The discrete frozen drops first appearing on the SHSs were triggered by tiny dusts falling on the surface before or during condensation icing. The key approach in delaying or resisting frost growth on SHSs with jumping condensates is to retard initial ice crystal formation, e.g., eliminating the edge effect and keeping the SHSs clean.

Do droplets always move following the wettability gradient

Impacting droplets on rough surfaces with a wettability gradient have been reported to rebound obliquely or migrate following the wettability gradient due to the unbalanced interfacial forces created by such heterogeneous architectures. Here we demonstrate under certain conditions, droplets can be self-propelled against the wettability gradient. We show that the local Cassie-to-Wenzel transition in the droplet is critical for such a counter-intuitive phenomenon. We believe that our letter extends our conventional understanding of wettability dynamics to heterogeneous surfaces and provides important insight for the design of micro/nanotextured surfaces for controlled droplet manipulation.

Visualizing millisecond chaotic mixing dynamics in microdroplets: A direct comparison of experiment and simulation

In order to fully explore and utilize the advantages of droplet-based microfluidics, fast, sensitive, and quantitative measurements are indispensable for the diagnosis of biochemical reactions in microdroplets. Here, we report an optical detection technique using two-photon fluorescence lifetime imaging microscopy, with an aligning-summing and non-fitting division method, to depict two-dimensional (2D) maps of mixing dynamics by chaotic advection in microdroplets with high temporal and spatial resolution. The mixing patterns of two dye solutions inside droplets were quantitatively and accurately measured. The mixing efficiency in a serpentine droplet mixer was also quantified and compared with the simulation data. The mapped chaotic mixing dynamics agree well with the numerical simulation and theoretical prediction. This quantitative characterization is potentially applicable to the real-time kinetic study of biological and chemical reactions in droplet-based microfluidic systems.

A facile microfluidic strategy for measuring interfacial tension

We report a facile method for measuring interfacial tension (IFT, γ) of immiscible fluids using a microfluidic device. The IFT is determined by monitoring the pressure drop across a microchannel, where a pair of modified Laplace sensors (formed by tapered channels) are connected, and the curvatures of the interfaces in the tapered channels. The method was tested with the model oil/water systems, and the results agreed well with a commercial tensiometry. We expect this method to be easily implemented in common microfluidic laboratories and supply a low-cost and fast way for interfacial tension measurement.

Quantitative imaging of mixing dynamics in microfluidic droplets using two-photon fluorescence lifetime imaging

Droplet-based microfluidic systems enable miniaturization of chemical reactions in femtoliter to picoliter volume compartments. Quantifying mixing dynamics of the reagents in droplets is critical to determine the system performance. In this Letter, we developed a two-photon excitation fluorescence lifetime imaging technique to quantitatively image the mixing dynamics in microfluidic droplets. A cross/autocorrelation method was used to reconstruct a high-quality fluorescence lifetime image of the droplet. The fluorescence decay was analyzed for accurate determination of the mixing ratio at each pixel of the image.

In vitro epithelial organoid generation induced by substrate nanotopography.

The extracellular matrix (ECM) exhibits tissue-specific topography and composition and plays a crucial role in initiating the biochemical and biomechanical signaling required for organizing cells into distinct tissues during development. How single cells assemble into structures featuring specific shapes in response to external cues is poorly understood. We examined the effect of substrate nanotopography on the morphogenesis of several types of epithelial cells and found that in response to the topography, Calu-3 and MDCK-II cells formed organoids that closely resemble their morphology in vivo. This finding represents the first demonstration that substrate nanotopography, one of the first physical cues detected by cells, can by itself induce epithelial tissue-like organization. Our results provide insights, in terms of a new aspect of ECM topography, into the design of future tissue-engineering systems and the study of mechanosignaling in the epithelium during normal development and tumor progression.

Real-time Monitoring of Hydrophobic Aggregation Reveals a Critical Role of Cooperativity in Hydrophobic Effect

The hydrophobic interaction drives nonpolar solutes to aggregate in aqueous solution, and hence plays a critical role in many fundamental processes in nature. An important property intrinsic to hydrophobic interaction is its cooperative nature, which is originated from the collective motions of water hydrogen bond networks surrounding hydrophobic solutes. This property is widely believed to enhance the formation of hydrophobic core in proteins. However, cooperativity in hydrophobic interactions has not been successfully characterized by experiments. Here, we quantify cooperativity in hydrophobic interactions by real-time monitoring the aggregation of hydrophobic solute (hexaphenylsilole, HPS) in a microfluidic mixer. We show that association of a HPS molecule to its aggregate in water occurs at sub-microsecond, and the free energy change is −5.8 to −13.6 kcal mol−1. Most strikingly, we discover that cooperativity constitutes up to 40% of this free energy. Our results provide quantitative evidence for the critical role of cooperativity in hydrophobic interactions.