Simulating Heat Transfer During Transient Dropwise Condensation on a Low-Thermal-Conductivity Substrate.

The instantaneous heat-transfer performance of a surface is dictated by the number and sizes of drops on the surface. While performance averaged over longer times is of interest from a technology standpoint, accurate simulation of the transient state is important in condenser design because the maximum heat rejection of the surface occurs in this range. Steady-state dropwise condensation can be thought of as a collection of transient dropwise condensation cycles occurring in parallel. Traditional simulation of dropwise condensation has focused on making comparisons with experimental drop-size distributions at later times, after the process has reached a statistical stationary phase where the heat transfer is lower. Understanding how to model and simulate transient dropwise condensation where a maximum in heat transfer occurs will help us design higher heat-rejecting surfaces. Additionally, a constant temperature difference between the steam and the surface below the drop is assumed. While often valid, there are some cases where this is not valid, and measuring the drop growth rate is required. We report a way to simulate transient dropwise condensation using a measured population averaged drop growth rate. The simulation reasonably predicts the time evolution of the number density of drops, fractional coverage, normalized condensate volume, and median drop radius for pendant mode dropwise condensation experiments on a cooled, horizontal, dodecyltrichlorosilane-coated glass surface. It was also found that assuming a constant temperature difference grossly underpredicts the heat transfer. Modification of the single-drop heat-transfer model to include substrate conduction and a thermal boundary layer shows that in the limit of low thermal conductivity the drop growth rate is constant for large drops. Additionally, a comparison between experiments and simulation shows that condensation might be initialized by nucleation onto fixed sites and then transitions to random nucleation as more sites become activated and more favorable. Understanding how a substrate's thermal properties affect the progression of dropwise condensation is important in determining the removal performance of the surface. With the commercialization of 3D printing, it is possible to fabricate low-cost, lightweight, plastic substrates with physical texturing for condensation applications where mass and cost savings are critical.

Adaptive adhesion by a beetle: manipulation of liquid bridges and their breaking limits.

A drop brought into contact with a nearby substrate can wet and spread against the substrate, forming a liquid bridge that exerts a capillary force. This force due to surface tension can be used to "grab" the substrate, pulling it toward the drop. "Wet" adhesion results from the parallel action of an array of small liquid bridges. The Florida palm beetle, Hemisphaerota cyanea, uses wet adhesion to defend itself against attacking predators by adhering to the palm leaf using an array of about 120,000 µm-sized liquid bridges. The beetle's survival depends on the strength of adhesion which, in turn, depends on how liquid bridges break. Individual bridges break when they go unstable, according to their response curves. However, the ultimate strength of an individual bridge depends on the class of disturbances to which it is subjected, and it has been speculated that the beetle may have some control over this class. The authors experimentally study families of liquid bridge equilibria for their breaking limits when subjected to constant-length (L) and constant-force (F) disturbances. While to control constant-L disturbances is straightforward, to apply and control constant-F disturbances on a liquid bridge requires more ingenuity. The authors introduce an apparatus with a lever-arm and a ball-bearing slide. The authors then compare our experimentally measured bridge response curves to the force trace from experiments on the beetle (prior literature) to infer the mode of beetle detachment. Under normal loads, the beetle detaches as a constant-L instability for smaller loads and as a constant-F instability for larger loads. The beetle's ability to adjust the type and magnitude of loading in real time is not only crucial to its survival but has implications for the design of various engineering devices.

Condensation on surface energy gradient shifts drop size distribution toward small drops.

During dropwise condensation from vapor onto a cooled surface, distributions of drops evolve by nucleation, growth, and coalescence. Drop surface coverage dictates the heat transfer characteristics and depends on both drop size and number of drops present on the surface at any given time. Thus, manipulating drop distributions is crucial to maximizing heat transfer. On earth, manipulation is achieved with gravity. However, in applications with small length scales or in low gravity environments, other methods of removal, such as a surface energy gradient, are required. This study examines how chemical modification of a cooled surface affects drop growth and coalescence, which in turn influences how a population of drops evolves. Steam is condensed onto a horizontally oriented surface that has been treated by silanization to deliver either a spatially uniform contact angle (hydrophilic, hydrophobic) or a continuous radial gradient of contact angles (hydrophobic to hydrophilic). The time evolution of number density and associated drop size distributions are measured. For a uniform surface, the shape of the drop size distribution is unique and can be used to identify the progress of condensation. In contrast, the drop size distribution for a gradient surface, relative to a uniform surface, shifts toward a population of small drops. The frequent sweeping of drops truncates maturation of the first generation of large drops and locks the distribution shape at the initial distribution. The absence of a shape change indicates that dropwise condensation has reached a steady state. Previous reports of heat transfer enhancement on chemical gradient surfaces can be explained by this shift toward smaller drops, from which the high heat transfer coefficients in dropwise condensation are attributed to. Terrestrial applications using gravity as the primary removal mechanism also stand to benefit from inclusion of gradient surfaces because the critical threshold size required for drop movement is reduced.