Dynamics of Nanoparticle Self-Assembly into Superhydrophobic Liquid Marbles during Water Condensation (original) (raw)

Dropwise Condensation Studies on Multiple Scales

Heat Transfer Engineering

Recent advances in nanotechnology, chemical/physical texturing and thin film coating technology generate definite possibilities for sustaining a dropwise mode of condensation for much longer durations than was previously possible. The availability of superior experimental techniques also leads to deeper understanding of the process parameters controlling the relevant transport phenomena, the distinguishing feature of which is the involvement of a hierarchy of length/time scales, proceeding from nuclei formation, to clusters, all the way to macroscopic droplet ensemble, drop coalescence, and subsequent dynamics. This paper is an attempt to connect and present a holistic framework of modeling and studying dropwise condensation at these multiple scales. After a review of the literature, discussions on the following problems are presented: (i) atomistic modeling of nucleation; (ii) droplet–substrate interaction; (iii) surface preparation; (iv) simulation of fluid motion inside sliding d...

Self-Sustained Cascading Coalescence in Surface Condensation

ACS Applied Materials & Interfaces, 2019

Sustained dropwise condensation of water requires rapid shedding of condensed droplets from the surface. Here, we elucidate a microfluidic mechanism that spontaneously sweeps condensed microscale droplets without the need for the traditional droplet removal pathways such as use of superhydrophobicity for droplet rolling and jumping and utilization of wettability gradients for directional droplet transport among others. The mechanism involves self-generated, directional, cascading coalescence sequences of condensed microscale droplets along standard hydrophobic microgrooves. Each sequence appears like a spontaneous zipping process, can sweep droplets along the microgroove at speeds of up to ∼1 m/s, and can extend for lengths more than 100 times the microgroove width. We investigate this phenomenon through high-speed in situ microscale condensation observations and demonstrate that it is enabled by rapid oscillations of a condensate meniscus formed locally in a filled microgroove and pinned on its edges. Such oscillations are in turn spontaneously initiated by coalescence of an individual droplet growing on the ridge with the microgroove meniscus. We quantify the coalescence cascades by characterizing the size distribution of the swept droplets and propose a simple analytical model to explain the results. We also demonstrate that, as condensation proceeds on the hydrophobic microgrooved surface, the coalescence cascades recur spontaneously through repetitive dewetting of the microgrooves. Lastly, we identify surface design rules for consistent realization of the cascades. The hydrophobic microgrooved textures required for the activation of this mechanism can be realized through conventional, scalable surface fabrication methods on a broad range of materials (we demonstrate with aluminum and silicon), thus promising direct application in a host of phasechange processes.

Condensation on Superhydrophobic Surfaces: The Role of Local Energy Barriers and Structure Length Scale

2012

Supporting Movie 1 Droplet growth and ejection captured using ESEM on the Si nanopillar surface shown in Figure 1e (݈ ൌ 2 µm, ‫ܧ‬ * ൌ 0.63). Coalescence-induced droplet ejection was observed towards the end of the video. Playback speed: 4x (MPEG; 28 MB). Field of view: 266 µm x 133 µm (width x height). Supporting Movie 2 Normal zipping behavior captured using OM on a Si nanopillar surface (݈ ൌ 300 nm, ‫ܧ‬ * ൌ 1.28). Playback speed: 1/8x (MPEG; 9.1 MB). Field of view: 176 µm x 132 µm (width x

Beads-assisted Investigation of Self-Coalescence of Sessile Water Drops

A simple strategy was proposed to investigate the self-coalescence of impurity water droplets at the early stage under quasi-equilibrium conditions. In this strategy, the droplets were allowed to coalesce on a cover glass in a closed cell to eliminate the effects of gravity and evaporation. Moreover, indicator beads were deliberately added into water to construct bead-containing droplets hence tracking the process of the fast coalescence of the water drops. In addition, surfactant was added in this coalescence system to decrease the viscosity of the water, which could slow down the coalescence process, making it possible to be captured by a fast camera. The surfactant was able to induce the formation of asymmetric droplets that are of irregular morphologies. Interestingly, such droplets of asymmetric shapes exhibited similar coalescing process, during which the relationship was obtained between the radius (rm) of the small bridge formed and time (t) consumed. It follows a scaling law of rm t1/2, similar to that of symmetric coalescence reported by previous researchers.

Growth of homogeneously nucleated water droplets: a quantitative comparison of experiment and theory

Atmospheric Research, 2003

The formation of aerosols proceeds through nucleation, growth and aging stages. The understanding of nucleation and droplet growth is essential for handling the more complex atmospheric condensation processes. To achieve this goal, measurements of the nucleation rate of various systems are performed in an expansion chamber. In this manner nucleation and growth are decoupled by applying a short nucleation pulse of about 1 ms during which the nuclei are formed. The subsequent droplet growth is quantitatively monitored by Mie-scattering. To this end, the Miemaxima and-minima are detected as a function of time and compared to theoretical Mie-scattering calculations for increasing radii. In this fashion, a wealth of growth curves for pure water depending on supersaturations, number densities of droplets, and temperatures were obtained. Following the

Kinetics of droplet growth processes: Simulations, theory, and experiments

1989

The formation of a distribution of various size droplets is a characteristic feature of many systems from thin films and breath figures to fog and clouds. In this paper we present the results of our investigations of the kinetics of droplet growth and coalescence. In general, droplet formation occurs either by spontaneous nucleation or by growth from heterogeneously distributed nucleation centers, such as impurities. We have introduced two models to describe these two types of processes. In the homogeneous nucleation model droplets can form and grow anywhere in the system. The results of the simulations of the model are presented and it is shown that the droplet size distribution has a bimodal structure consisting of a monodispersed distribution of large droplets superimposed on a polydispersed distribution of smaller droplets. A scaling description for the evolution of the timedependent droplet size distribution and its moments is presented and it is found that the scaling predictions are in excellent agreement with the simulations. A rate-equation similar to the Smoluchowski equation is also introduced for describing the kinetics of homogeneous droplet growth. The results of the simulations of the homogeneous nucleation model are also compared with the experiments on droplet growth in thin films obtained by vapor deposition of tin on sapphire substrate. It appears that this model captures the essential features of the distribution of droplets in the vapor deposition experiments. We also introduce a heterogeneous nucleation model for studying processes in which droplets only form and grow at certain nucleation centers which are initially chosen at random. Simulations, scaling theory, and a kinetic equation approach for describing the heterogeneously nucleated droplet growth model are also presented. The theoretical predictions are found to be in excellent agreement with the simulations.

Coalescence of Pendant Droplets on an Inclined Super-hydrophobic Substrate

Dynamics of micro droplets, coalescence underneath an inclined hydrophobic surface is explored, experimentally as well as computationally. Such a situation often arises during dropwise condensation, atmospheric dew formation, condensation in green houses, and metal vapor condensation during enrichment process, etc. Coalescence induced instability in the pendant mode is an effective means of passively enhancing heat transfer coefficient during dropwise condensation. Inclined substrates have natural advantages in terms of rendering effective passive sweeping of drops from the substrate, thereby exposing fresh preferred sites for renucleation. As compared to coalescences of sessile droplets, pendant mode induces flow instabilities at a much faster rate, thereby enhancing the associated heat/ mass transport characteristics. Against this background, the present study reports an experiment involving the coalescence of water drops in pendant mode on an inclined hydrophobic copper substrate of size 20 mm × 20 mm × 2 mm. The substrate has been prepared by developing a monolayer of n-octadecanethiol on it by chemical action. This work focuses on fundamental understanding of the mechanism of drop coalescence underneath an inclined superhydrophobic substrate to provide aid in controlling and promoting dropwise mode of condensation in order to increase efficiency of condenser or to aid in enrichment of heavy liquid metals in closed vacuum condition. The coalescence process gets initiated by the extra available surface energy which gets released in the process. Immediately afterwards, the process of oscillating free surface formed during the coalescence is limited by viscous and inertia forces. The free surface oscillations can last ~ 10 to 100 milliseconds, depending on the size of droplets and their thermophysical properties. Depending on the local wettability, size of drops which are coalescing, substrate inclination and pinning/de-pinning behavior, droplet instability conditions are generated, which result in its slide-off motion on the substrate.

Droplet Growth and Transition to Coalescence in Confined Geometries

Langmuir, 2006

A thermodynamic theory is developed to predict growth, rearrangement to a close-packed ensemble, and transition to a deformed or coalesced state for droplets in a confined space. For the close-packed configuration, analysis of forced interactions between confined droplets yields analytical criteria for predicting whether droplets will deform, and if they will coalesce. Relevant nondimensional parameters are identified to generalize results in terms of energy barrier maps, and their use for predicting interacting droplet behavior is described.

A New Mechanism of Droplet Size Distribution Broadening during Diffusional Growth

Journal of the Atmospheric Sciences, 2013

A new mechanism has been developed for size distribution broadening toward large droplet sizes. This mechanism may explain the rapid formation of large cloud droplets, which may subsequently trigger precipitation formation through the collision-coalescence process. The essence of the new mechanism consists of a sequence of mixing events between ascending and descending parcels. When adiabatically ascending and descending parcels having the same initial conditions at the cloud base arrive at the same level, they will have different droplet sizes and temperatures, as well as different supersaturations. Isobaric mixing between such parcels followed by further ascents and descents enables the enhanced growth of large droplets. The numerical simulation of this process suggests that the formation of large 30-40-mm droplets may occur within 20-30 min inside a shallow adiabatic stratiform layer. The dependencies of the rate of the droplet size distribution broadening on the intensity of the vertical fluctuations, their spatial amplitude, rate of mixing, droplet concentration, and other parameters are considered here. The effectiveness of this mechanism in different types of clouds is discussed.

Multimode Multidrop Serial Coalescence Effects during Condensation on Hierarchical Superhydrophobic Surfaces

Langmuir, 2013

The prospect of enhancing the condensation rate by decreasing the maximum drop departure diameter significantly below the capillary length through spontaneous drop motion has generated significant interest in condensation on superhydrophobic surfaces (SHS). The mobile coalescence leading to spontaneous drop motion was initially reported to occur only on hierarchical SHS, consisting of both nanoscale and microscale topological features. However, subsequent studies have shown that mobile coalescence also occurs on solely nanostructured SHS. Thus, recent focus has been on understanding the condensation process on nanostructured surfaces rather than on hierarchical SHS. In this work, we investigate the impact of microscale topography of hierarchical SHS on the droplet coalescence dynamics and wetting states during the condensation process. We show that isolated mobile and immobile coalescence between two drops, almost exclusively focused on in previous studies, are rare. We identify several new droplet shedding modes, which are aided by tangential propulsion of mobile drops. These droplet shedding modes comprise of multiple droplets merging during serial coalescence events, which culminate in formation of a drop that either departs or remains anchored to the surface. We directly relate postmerging drop adhesion to formation of drops in nanoscale as well as microscale Wenzel and Cassie−Baxter wetting states. We identify the optimal microscale feature spacing of the hierarchical SHS, which promotes departure of the highest number of microdroplets. This optimal surface architecture consists of microscale features spaced close enough to enable transition of larger droplets into micro-Cassie state yet, at the same time, provides sufficient spacing in-between the features for occurrence of mobile coalescence.