Tom Krupenkin

Design of ice-free nanostructured surfaces based on repulsion of impacting water droplets

Materials that control ice accumulation are important to aircraft efficiency, highway and powerline maintenance, and building construction. Most current deicing systems include either physical or chemical removal of ice, both energy and resource-intensive. A more desirable approach would be to prevent ice formation rather than to fight its build-up. Much attention has been given recently to freezing of static water droplets resting on supercooled surfaces. Ice accretion, however, begins with the droplet/substrate collision followed by freezing. Here we focus on the behavior of dynamic droplets impacting supercooled nano- and microstructured surfaces. Detailed experimental analysis of the temperature-dependent droplet/surface interaction shows that highly ordered superhydrophobic materials can be designed to remain entirely ice-free down to ca. -25 to -30 °C, due to their ability to repel impacting water before ice nucleation occurs. Ice accumulated below these temperatures can be easily removed. Factors contributing to droplet retraction, pinning and freezing are addressed by combining classical nucleation theory with heat transfer and wetting dynamics, forming the foundation for the development of rationally designed ice-preventive materials. In particular, we emphasize the potential of hydrophobic polymeric coatings bearing closed-cell surface microstructures for their improved mechanical and pressure stability, amenability to facile replication and large-scale fabrication, and opportunities for greater tuning of their material and chemical properties.

Reverse electrowetting as a new approach to high- power energy harvesting

Over the last decade electrical batteries have emerged as a critical bottleneck for portable electronics development. High-power mechanical energy harvesting can potentially provide a valuable alternative to the use of batteries, but, until now, a suitable mechanical-to-electrical energy conversion technology did not exist. Here we describe a novel mechanical-to-electrical energy conversion method based on the reverse electrowetting phenomenon. Electrical energy generation is achieved through the interaction of arrays of moving microscopic liquid droplets with novel nanometer-thick multilayer dielectric films. Advantages of this process include the production of high power densities, up to 103 W m−2; the ability to directly utilize a very broad range of mechanical forces and displacements; and the ability to directly output a broad range of currents and voltages, from several volts to tens of volts. These advantages make this method uniquely suited for high-power energy harvesting from a wide variety of environmental mechanical energy sources.

Predictive model for ice formation on superhydrophobic surfaces.

The prevention and control of ice accumulation has important applications in aviation, building construction, and energy conversion devices. One area of active research concerns the use of superhydrophobic surfaces for preventing ice formation. The present work develops a physics-based modeling framework to predict ice formation on cooled superhydrophobic surfaces resulting from the impact of supercooled water droplets. This modeling approach analyzes the multiple phenomena influencing ice formation on superhydrophobic surfaces through the development of submodels describing droplet impact dynamics, heat transfer, and heterogeneous ice nucleation. These models are then integrated together to achieve a comprehensive understanding of ice formation upon impact of liquid droplets at freezing conditions. The accuracy of this model is validated by its successful prediction of the experimental findings that demonstrate that superhydrophobic surfaces can fully prevent the freezing of impacting water droplets down to surface temperatures of as low as -20 to -25 °C. The model can be used to study the influence of surface morphology, surface chemistry, and fluid and thermal properties on dynamic ice formation and identify parameters critical to achieving icephobic surfaces. The framework of the present work is the first detailed modeling tool developed for the design and analysis of surfaces for various ice prevention/reduction strategies.

Controlled switching of the wetting behavior of biomimetic surfaces with hydrogel-supported nanostructures

An important feature of biological systems is their response to external stimuli with subsequent changes in properties and function. The ability to “engineer” adaptiveness into next-generation materials is becoming a key requirement and challenge in chemistry, materials science and engineering. Recently we have described new hybrid nano/microstructures capable of dynamic actuation by a hydrogel “muscle”. Here we demonstrate the application of a variation of such biomimetic surfaces in controlled reversible switching of the surface wetting behavior. Arrays of rigid nanostructures were integrated with responsive hydrogel films by performing in situ polymerization in microscopic confinement of two surfaces. The attachment of hydrogel was achieved through a multifunctional polymeric anchoring layer. Using two different attachment strategies, several designs involving an array of either attached or free-standing nanocolumns embedded in the hydrogel film are described. We demonstrate a superhydrophobic–hydrophilic transition (so-called “direct response”) or a hydrophilic–superhydrophobic transition (“reverse response”), respectively, upon the exposure of these two structures to water. We show that all the changes in the wetting behavior are reversible and the structures return to their original superhydrophobic or hydrophilic state upon drying. The ability to design surfaces with reversible changes in their wetting behavior may have exciting applications as “smart,” responsive materials with tunable water-repelling or water-attracting properties.

Dynamic tuning of optical waveguides with electrowetting pumps and recirculating fluid channels

We have developed an approach for using electrowetting actuation in recirculating fluidic channels to achieve dynamic tuning of optical fiber structures. The electrically controlled and fully reversible motion of the fluids and lubricants in these channels alters the refractive index profile experienced by the optical waveguide modes of the fiber. When combined with in-fiber gratings and etched fibers, this fluidic system yields dynamically adjustable narrow and broadband fiber filters, respectively. The nonmechanical operation of these systems, their ability to support switching speeds on the order of milliseconds, and their excellent optical characteristics indicate a promising potential for electrowetting-actuated fluidic tuning in optical fiber devices and other photonic components.

Tunable Liquid Optics : Electrowetting-Controlled Liquid Mirrors Based on Self-Assembled Janus Tiles

In this paper, we describe a tunable, high-reflectivity optofluidic device based on self-assembly of anisotropically functionalized hexagonal micromirrors (Janus tiles) on the surface of an oil droplet to create a concave liquid mirror. The liquid mirror is deposited on a patterned transparent electrode that allows the focal length and axial position to be electrically controlled. The mirror is mechanically robust and retains its integrity even at high levels of vibrational excitation of the interface. The use of reflection instead of refraction overcomes the limited available refractive-index contrast between pairs of density-matched liquids, allowing stronger focusing than is possible for a liquid lens of the same geometry. This approach is compatible with optical instruments that could provide novel functionality-for example, a dynamic 3D projector, i.e., a light source which can scan an image onto a moving, nonplanar focal surface. Janus tiles with complex optical properties can be manufactured using our approach, thus potentially enabling a wide range of novel optical elements.

Tunable optical fiber devices based on broadband long-period gratings and pumped microfluidics

This letter describes classes of tunable microfluidic fiber (μFF) devices that use specially designed long-period gratings in which the phase matching condition is satisfied over a wide spectral range. Dynamic tuning is achieved by electrowetting-based pumping of microfluidic plugs back and forth over the gratings. As specific examples, we demonstrate dynamically tunable broadband attenuators and filters with adjustable profiles by using fluids with different refractive indices. These devices have attractive features that include in-fiber design and polarization-independent behavior together with low-power, nonmechanical, fully reversible, and latchable tuning.

Electrically tunable superhydrophobic nanostructured surfaces

In this work, we discuss dynamic electrical control of the wetting behavior of liquids on nanostructured surfaces spanning the entire possible range from superhydrophobic behavior to nearly complete wetting. It is demonstrated that a droplet of liquid can be reversibly switched between the superhydrophobic “rolling ball” state and the hydrophilic immobile droplet state by the application of electrical voltage and current. The nature of the transition mechanism is studied both experimentally and theoretically. The reported results provide novel methods of manipulating liquids at microscale.

Superhydrophobic membranes with electrically controllable permeability and their application to “smart” microbatteries

Electrically tunable membranes with controllable permeability have been experimentally demonstrated by combining nanostructured and microstructured superhydrophobic surfaces with the phenomenon of electrowetting. Electrowetting allows dynamical tuning of the contact angle that the liquid forms with the membrane nanofeatures and microfeatures, thus controlling the flow of the liquid through the membrane and, therefore, tuning the permeability of the entire structure. “Smart” electrochemical energy storage cells that can be activated on demand have been built by combining these membranes and microfabricated Zn∕MnO2 electrodes. A typical open-circuit voltage of 1.55V and capacity of 200μAh∕cm2 have been demonstrated.

Bubbler: A Novel Ultra-High Power Density Energy Harvesting Method Based on Reverse Electrowetting

We have proposed and successfully demonstrated a novel approach to direct conversion of mechanical energy into electrical energy using microfluidics. The method combines previously demonstrated reverse electrowetting on dielectric (REWOD) phenomenon with the fast self-oscillating process of bubble growth and collapse. Fast bubble dynamics, used in conjunction with REWOD, provides a possibility to increase the generated power density by over an order of magnitude, as compared to the REWOD alone. This energy conversion approach is particularly well suited for energy harvesting applications and can enable effective coupling to a broad array of mechanical systems including such ubiquitous but difficult to utilize low-frequency energy sources as human and machine motion. The method can be scaled from a single micro cell with 10−6 W output to power cell arrays with a total power output in excess of 10 W. This makes the fabrication of small light-weight energy harvesting devices capable of producing a wide range of power outputs feasible.