Controlling Interfaces: Liquid Bridge Stability And Transient Dropwise Condensation

Abstract

Controlling the liquid-gas interface position is important for various applications. We focus on two applications: manipulating dropwise condensation and breaking liquid bridges. In dropwise condensation, removal of drops from a condensing surface is needed to prevent the transition to filmwise condensation, generally undesirable because of lower heat transfer performance. Successful removal of drops from a surface can be achieved by manipulation of the drop interface through mechanical oscillation of the surface [91, 28], electrowetting [23, 76], temperature gradients [40, 103], surface energy gradients by means of chemical treatments [150, 27, 21, 24, 68, 125, 126] or physical texturing [64, 77, 148], or gravity [44, 57]. A combination of methods can also be employed. For liquid bridges, manipulation of the interface can be achieved by mechanical loading. Applications such as the mounting of surveillance cameras, the massive parallel manipulation of mm-sized objects, and the grabbing-andreleasing of a single substrate require use of an adhesion device. The SECAD (switchable electronically-controlled capillary adhesion device) consists of several liquid bridges in parallel that can either be addressed all at once [139, 138], or individually [99, 98]. In these applications, there are periods where bridge stabilization is required (e.g., holding a surveillance camera against the ceiling, holding onto small objects or a substrate). There are also periods where bridge destabilization (bridge breaking) is required (e.g., unmounting a cam- era, releasing small objects or a substrate). From dropwise condensation experiments on uniform contact-angle and surface energy gradient surfaces, we learned that the shift in drop-size distributions towards smaller drops by sweeping could explain the observed increase in heat transfer performance of a surface energy gradient surface [84]. From simulating transient dropwise condensation, we learned that using the measured single drop growth rate versus a constant temperature difference between the steam and the surface is needed to get agreement between simulation and experiments conducted on low thermal conductivity surfaces. From mechanically loading liquid bridges with constantvolume, -force, and -length loadings and measuring their response, we learn that the palm beetle's mode of detachment depends on the type (i.e., continuous vs. discrete) and the magnitude of loading [85]. Part I: A held 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 micrometer-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. We experimentally study families of liquid bridge equilibria for their break- ing limits when subjected to constant-length (L) and constant-force (F ) disturbances. While controlling constant-L disturbances is straightforward, to apply and control constant-F disturbances on a liquid bridge requires more ingenuity. To achieve this we introduce an apparatus with a lever-arm and a ball-bearing slide. We then compare our experimentally measured bridge response curves to the force trace from experiments on the beetle [35] 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. Part II: 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 means sweeping or dripping of larger drops, which 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 the underside of 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 that for 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 distribution 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. Higher heat transfer coefficients in dropwise condensation are attributed to smaller median drop size. 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. We also simulate the entire transient portion of dropwise condensation with a Matlab routine. While statistical steady-state condensation at longer times is of interest from a technology standpoint, accurate simulation of the transient state is important to understanding steady-state. Steady-state dropwise condensation is really a statistical state consisting of 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 single instants in time, typically at later times, after the process has reached a statistical steady-state. Additionally, a constant temperature difference between the steam and the substrate is assumed. While this assumption is valid for metal surfaces possessing a high thermal conductivity, this assump- tion is not necessarily true for low thermal conductivity surfaces such as glass. We report a way to simulate the entire transient portion of dropwise condensation using a population averaged isolated drop growth rate measured directly from experiment using single drop tracking to grow the drops. The simulation reasonably predicts the time evolution of the number density of drops, the fractional coverage, the normalized condensate volume, and the median drop radius for condensation experiments performed on a horizontal hydrophobic surface exposed to coolant temperatures of 1? C, 3? C, 30? C, and 50? C. In the case of a glass substrate chemically coated with dodecyltrichlorosilane, it was found that use of a constant temperature difference grossly under predicted the heat transfer. Additionally, the amount of liquid condensed on the surface was independent of the cooling temperature for the coolant temperature range investigated. This has important implications for energy costs in heat transfer applications.