Controlling Condensation of Water Using Hybrid Hydrophobic-Hydrophilic Surfaces

Heterogeneous nucleation of water plays an important role in wide range of natural and industrial processes. Though heterogeneous nucleation of water is ubiquitous and everyday experience, spatial control of this important phenomenon is extremely difficult. Here we show, for the first time, that spatial control in the heterogeneous nucleation of water can be achieved by manipulating the local nucleation energy barrier and nucleation rate via the modification of the local intrinsic wettability of a surface by patterning hybrid hydrophobic-hydrophilic regions on a surface. Such ability to control water nucleation could address the condensation-related limitations of superhydrophobic surfaces, and has implications for efficiency enhancements in energy and desalination systems.General Electric Company. Global Research Center (Nanotechnology Program)Massachusetts Institute of Technology. d’Arbeloff Career Development Chai

Creating nanoscale emulsions using condensation

Nanoscale emulsions are essential components in numerous products, ranging from processed foods to novel drug delivery systems. Existing emulsification methods rely either on the breakup of larger droplets or solvent exchange/inversion. Here we report a simple, scalable method of creating nanoscale water-in-oil emulsions by condensing water vapor onto a subcooled oil-surfactant solution. Our technique enables a bottom-up approach to forming small-scale emulsions. Nanoscale water droplets nucleate at the oil/air interface and spontaneously disperse within the oil, due to the spreading dynamics of oil on water. Oil-soluble surfactants stabilize the resulting emulsions. We find that the oil-surfactant concentration controls the spreading behavior of oil on water, as well as the peak size, polydispersity, and stability of the resulting emulsions. Using condensation, we form emulsions with peak radii around 100 nm and polydispersities around 10%. This emulsion formation technique may open different routes to creating emulsions, colloidal systems, and emulsion-based materials

Vibration damping using low-wave-speed media with applications to precision machines

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2004.Includes bibliographical references (p. 178-182).Vibration and noise are an ever-present problem in the majority of mechanical systems, from consumer products to precision manufacturing systems. But most approaches for vibration suppression are expensive and invasive, so only a small subset of the techniques developed in research labs are widely used. In this thesis, we present a novel wave-based damping approach for the suppression of vibration in machines and structures. Our studies show that significant broad-band damping can be attained with little added mass via dynamic interaction between a structure and a low-density, low-wave-speed medium (such as a foam or powder). This damping phenomenon has great promise for many applications because it is robust (that is, not tuned), does not introduce significant creep into a structure, can accommodate large strains, and can be realized using materials that are light weight, low cost, durable, insensitive to temperature, and easy to package. We report on several experiments in which flexural and longitudinal vibration are attenuated using this approach. Experiments on flexural vibration of structures filled with low-density powder show that high damping is obtained (with loss factors as high as 12 percent for a powder fill whose mass is 2.3 percent of that of the beam) over a broad frequency range. Somewhat surprisingly, the response is found to be linear over a wide range of amplitudes. We propose that the powder can be modeled as a fluid in which pressure waves can propagate and find that such a model matches the experiments well. These findings suggest that any moderately lossy medium in which the speed of wave propagation is sufficiently low can be used to obtain similar responses.(cont.) We find that low-density foams coupled to structures exhibit com- parable attenuations over a somewhat broader frequency range, and that the responses can be accurately predicted if dilatation and shear waves are included in the model. We develop simplified models for these phenomena, and thence obtain guidelines for design of structures incorporating low-wave-speed media. The approach is compared to other damping techniques, and applications to belt- driven positioning systems and precision flexure assemblies are described. .by Kripa K. Varanasi.Ph.D

Critical heat flux maxima during boiling crisis on textured surfaces

Enhancing the critical heat flux (CHF) of industrial boilers by surface texturing can lead to substantial energy savings and global reduction in greenhouse gas emissions, but fundamentally this phenomenon is not well understood. Prior studies on boiling crisis indicate that CHF monotonically increases with increasing texture density. Here we report on the existence of maxima in CHF enhancement at intermediate texture density using measurements on parametrically designed plain and nano-textured micropillar surfaces. Using high-speed optical and infrared imaging, we study the dynamics of dry spot heating and rewetting phenomena and reveal that the dry spot heating timescale is of the same order as that of the gravity and liquid imbibition-induced dry spot rewetting timescale. Based on these insights, we develop a coupled thermal-hydraulic model that relates CHF enhancement to rewetting of a hot dry spot on the boiling surface, thereby revealing the mechanism governing the hitherto unknown CHF enhancement maxima.MIT Shapiro FellowshipChevron CorporationKuwait-MIT Center for Natural Resources and the Environmen

Separating Oil-Water Nanoemulsions using Flux-Enhanced Hierarchical Membranes

Membranes that separate oil-water mixtures based on contrasting wetting properties have recently received significant attention. Separation of nanoemulsions, i.e. oil-water mixtures containing sub-micron droplets, still remains a key challenge. Tradeoffs between geometric constraints, high breakthrough pressure for selectivity, high flux, and mechanical durability make it challenging to design effective membranes. In this paper, we fabricate a hierarchical membrane by the phase inversion process that consists of a nanoporous separation skin layer supported by an integrated microporous layer. We demonstrate the separation of water-in-oil emulsions well below 1 μm in size. In addition, we tune the parameters of the hierarchical membrane fabrication to control the skin layer thickness and increase the total flux by a factor of four. These simple yet robust hierarchical membranes with engineered wetting characteristics show promise for large-scale, efficient separation systems.MIT Energy InitiativeShell Oil CompanyMIT Energy Initiative (Fellowship

Hierarchical Superhydrophobic Surfaces Resist Water Droplet Impact

URL to paper listed on conference siteIn this paper, we present static and dynamic wetting interactions of water droplets on a variety of superhydrophobic surfaces. For sessile droplets, wetting states were determined by measuring contact angles and comparing them to that obtained from equilibrium Cassie and Wenzel states. Surprisingly, we find that roll-off angles are minimized on surfaces expected to induce Wenzel-like wetting in equilibrium. We argue that droplets on these surfaces are metastable Cassie droplets whose internal Laplace pressure is insufficient to overcome the capillary pressure resulting from the energy barrier required to completely wet the posts. In the case of impacting droplets the water hammer and Bernoulli pressures must be compared with the capillary pressure. Experiments with impacting droplets using a high-speed camera and specific surface textures that can delineate various wetting regimes show very good agreement with this simple pressurebalance model. These studies show that hierarchical micronano surfaces are optimum for droplet impact resistance.GE Global Research Cente

Enhancing droplet deposition through in-situ precipitation

Retention of agricultural sprays on plant surfaces is an important challenge. Bouncing of sprayed pesticide droplets from leaves is a major source of soil and groundwater pollution and pesticide overuse. Here we report a method to increase droplet deposition through in-situ formation of hydrophilic surface defects that can arrest droplets during impact. Defects are created by simultaneously spraying oppositely charged polyelectrolytes that induce surface precipitation when two droplets come into contact. Using high-speed imaging, we study the coupled dynamics of drop impact and surface precipitate formation. We develop a physical model to estimate the energy dissipation by the defects and predict the transition from bouncing to sticking. We demonstrate macroscopic enhancements in spray retention and surface coverage for natural and synthetic non-wetting surfaces and provide insights into designing effective agricultural sprays