Micro and Nano Fluidics Research Group

Nanotechnology Cluster and Mechanical Engineering
University of Wisconsin - Madison

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In this work, we developed a new type of three-dimensional nanoscale surface topography, which we refer to as “nanonails,” that dramatically alters the wetting behavior of nanostructured substrates. This approach makes it possible to create superlyophobic substrates – surfaces that resist wetting by virtually any liquid. This is achieved by “locking” the liquid in a metastable non-wetting state that causes the substrate to “repel” the liquid even if the analogous flat surface by itself is highly wettable. We demonstrated this approach experimentally by transforming ordinary fluoropolymer surfaces, which are readily wetted by the majority of common low-surface-tension liquids, into superlyophobic substrates that completely repel an extremely broad range of liquids with different chemical natures (alcohols, ethers, esters, water-alcohol mixtures, aromatic hydrocarbons, silicon oils) and with surface tensions ranging from 21.8 mN/m (ethanol) to 72 mN/m (water). A. Ahuja, J.A. Taylor, V. Lifton, A. Sidorenko, T. Salamon, E. Lobaton, P. Kolodner, and T. Krupenkin, “Nanonails – A Simple Geometrical Approach to Electrically Tunable Superlyophobic Surfaces,” Langmuir, 24, 9–14 (2008). nails


Nanograss represents a novel type of nanostructured surface with dynamically tunable wetting behavior. The state of liquid droplets on a nanograss surface can be dynamically and reversibly changed all the way from a highly mobile rolling-ball state to almost complete wetting by the application of a small voltage and current. We have examined experimentally and theoretically the evolution of the droplet states and their dependence on the details of the nanograss geometry. The pro¬posed approach can potentially enable novel methods of manipulating microscopically small volumes of liquids. This includes low-friction liquid transport, the ability to precisely control droplet shape and posi¬tion, as well as dynamic control over the penetration of liquids through the nanostructured layer. The obtained results potentially open new and exciting opportunities in microfluidics, chemical micro¬reactors, bio/chemical detection and characterization, optics, drag reduc¬tion, and many other areas.
The work has attracted substantial media attention including Physics Today (2004), The New York Times (2004 ) and ABC News (2005). The results of the work are described in 7 publications, one granted and 12 pending patents.

Nanostructured microbattery-on-a-chip

One of the well-known and long-standing problems in electrical batteries is self-discharge caused by a slow chemical reaction between the electrolyte and electrodes that occurs in the battery during its period of inactivity. This problem becomes especially acute in high-power-density batteries, such as those used by military and emergency systems. Traditionally, these applications employ so-called reserve batteries, where electrolyte is stored in a separate compartment and brought into contact with electrodes only upon battery activation shortly before use. Existing reserve batteries are expensive and do not allow scaling to micro size. We have employed “nanograss” technology to create a novel, highly-scalable and inexpensive reserve microbattery-on-a-chip. The developed battery can be considered as a particular example of the nanograss-based chemical microreactor that exploits controllable initiation of the chemical reaction using penetration of the liquid reagent through the nanostructured layer. In this case, the liquid reagent is a battery electrolyte, and the solid reagent is a battery electrode, which is separated from electrolyte by the nanograss layer. Upon the battery activation, the electrolyte is allowed to penetrate the nanograss and wet the electrode, thus initiating the electrochemical reaction. We believe that this type of battery can find a wide range of applications, including self-powered chips, RF tags, and unattended ground sensor networks.

This work has attracted substantial venture capital funding (over $1M per year for three years), has enjoyed considerable media attention including Scientific American (2006) and ABC News (2006), and has won an Excellence in Research award (2006). The results of the work are described in six pending patents, and two papers.

Electrowetting-actuated tunable liquid microlens

Tunable minimal-energy surfaces such as those formed by liquid-gas and liquid-liquid interfaces exhibit a slew of interesting and unusual optical properties. They can provide unparalleled opportunities in developing novel optical devices that combine highly sophisticated optical processing capabilities with the intrinsic simplicity, high reliability, and low cost of droplet-based microfluidic devices. In this work, we proposed and experimentally demonstrated a tunable liquid microlens, which consists of a droplet of transparent conductive liquid placed on a low surface energy dielectric substrate with underlying electrodes. By varying the voltage applied to the structure, both the position and curvature of the microlens can be changed using electrowetting. As a result, the precise positioning of the microlens focal spot in all three dimensions is achieved. This translates into focusing, pan, tilt, and zoom abilities in imaging applications, or light beam steering and focusing in light control applications. If desired, the position and curvature of the microlens can be permanently fixed at any stage by employing a polymerizable liquid that can be rapidly solidified in response to an external stimulus, such as UV irradiation. The dependence of the microlens behavior on the properties of the materials involved is experimentally investigated and supported by theoretical calculations. Potential limitations of the microlens performance associated with the contact-angle hysteresis and stick/slip phenomena are outlined, and possible ways to alleviate them are assessed.
The work has attracted considerable media attention, including New Scientist (2002) and Science News (2003). The results of the work are described in two publications, 11 granted patents.

Fiber-based electrowetting-actuated optofluidic devices

Propagation of light through fiber- and waveguide- based optical devices can be substantially influenced by the optical properties of the surrounding media. Such devices naturally lend themselves to microfluidic-based control, which provides a number of important advantages over existing approaches, including very low power consumption, simple thermal management, and a wide range of tunability. In this work, we developed a new class of tunable microfluidic fiber 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 a number of attractive features that include in-fiber design and polarization-independent behavior together with low-power, nonmechanical, fully reversible, and latchable tuning.
The work won the 2003 Emerging Technologies Award. The results of the work are described in two publications and two patents.
optofluidic devices
© 2009-2013 Krupenkin Research Group, Mechanical Engineering, University of Wisconsin - Madison
Web master Elena Krupenkin.