The dynamics and stability of thin liquid films have fascinated scientists over many decades: the observations of regular wave patterns in film flows down a windowpane or along guttering, the patterning of dewetting droplets, and the fingering of viscous flows down a slope are all examples that are familiar in daily life. Thin film flows occur over a wide range of length scales and are central to numerous areas of engineering, geophysics, and biophysics; these include nanofluidics and microfluidics, coating flows, intensive processing, lava flows, dynamics of continental ice sheets, tear-film rupture, and surfactant replacement therapy. These flows have attracted considerable attention in the literature, which have resulted in many significant developments in experimental, analytical, and numerical research in this area. These include advances in understanding dewetting, thermocapillary- and surfactant-driven films, falling films and films flowing over structured, compliant, and rapidly rotating substrates, and evaporating films as well as those manipulated via use of electric fields to produce nanoscale patterns. These developments are reviewed in this paper and open problems and exciting research avenues in this thriving area of fluid mechanics are also highlighted.

Dynamics and stability of thin liquid films

When droplets coalesce on a superhydrophobic nanostructured surface, the resulting droplet can jump from the surface due to the release of excess surface energy. If designed properly, these superhydrophobic nanostructured surfaces can not only allow for easy droplet removal at micrometric length scales during condensation but also promise to enhance heat transfer performance. However, the rationale for the design of an ideal nanostructured surface as well as heat transfer experiments demonstrating the advantage of this jumping behavior are lacking. Here, we show that silanized copper oxide surfaces created via a simple fabrication method can achieve highly efficient jumping-droplet condensation heat transfer. We experimentally demonstrated a 25% higher overall heat flux and 30% higher condensation heat transfer coefficient compared to state-of-the-art hydrophobic condensing surfaces at low supersaturations (<1.12). This work not only shows significant condensation heat transfer enhancement but also promises a low cost and scalable approach to increase efficiency for applications such as atmospheric water harvesting and dehumidification. Furthermore, the results offer insights and an avenue to achieve high flux superhydrophobic condensation.

Jumping-Droplet-Enhanced Condensation on Scalable Superhydrophobic Nanostructured Surfaces

Introduction to heat transfer

A view of contact angle hysteresis from the perspectives of the three-phase contact line and of the kinetics of contact line motion is given. Arguments are made that advancing and receding are discrete events that have different activation energies. That hysteresis can be quantified as an activation energy by the changes in interfacial area is argued. That this is an appropriate way of viewing hysteresis is demonstrated with examples.

Contact angle hysteresis explained

Heat transfer enhancement for thermal energy storage using metal foams embedded within phase change materials (PCMs)

The present experimental study investigates the impact of porous fins on the pressure drop and heat transfer characteristics in plate-fin heat exchangers. Systematic experiments have been carried out in a simplified model of a plate-porous fin heat exchanger at a controlled test environment. The porous fins are made of 6101 aluminum-alloy foam materials with different permeabilities and porosities. Comparison of performance between the porous fins and the conventional louvered fins has been made. The experimental results indicate that friction and heat transfer rate are significantly affected by permeability as well as porosity of the porous fin. The porous fins used in the present study show similar thermal performance to the conventional louvered fin. However, the louvered fin shows a little better performance in terms of pressure drop. For compactness of the heat exchanger, the porous fins with high pore density and low porosity are preferable. Useful correlations for the friction factor and the modified j-factor are also given for the design of a plate-porous fin heat exchanger.

Flow and heat transfer correlations for porous fin in a plate-fin heat exchanger

A liquid drop that partially wets a solid surface will slide down the plane when it is tilted beyond a critical inclination. Here we report the study of the sliding velocity of such a drop. Experiments for measuring the steady sliding velocity of different liquids of drops are performed. We then construct a scaling law that predicts the sliding velocity given the physical properties, wetting characteristics, and size of the drop. When the sliding velocity is low and the drop distortion due to inclination is small, the scaling law is shown to correctly model the functional dependency of the measured sliding velocity.

Byung Ha Kang

Papers

Flow and heat transfer correlations for porous fin in a plate-fin heat exchanger

Sliding of liquid drops down an inclined solid surface.

Forced convection from aluminum foam materials in an asymmetrically heated channel

Enhancement of natural convection and pool boiling heat transfer via ultrasonic vibration

Heat transfer in the thermally developing region of a pulsating channel flow