Nanoengineered Surfaces

Wettability and Microfluidics -- Interactions between liquids and solids are ubiquitous in our physical environment and are typically characterized by the wetting angle that a liquid droplet makes on the solid surface. While wettability on flat and homogenous surfaces has been researched quite extensively, recent advances in micro-/nano-fabrication and coating technologies have enabled the development of smart engineered surfaces. Fundamental understanding of the wetting and liquid propagation behavior on these surfaces is important for a range of applications such as microfluidics, thermal management, lab-on-a-chip, water harvesting, optical, and biological systems.

At the Device Research Laboratory, we are working towards developing a better understanding of the change in wettability due to surface engineering, chemical heterogeneities, and in the presence of liquid-vapor phase change phenomena. These studies are critical for elucidating the underlying physical mechanisms behind the other research topics being investigated in the lab. For example, hydrophilic surfaces made superhydrophilic due to structuring have resulted in enhancement of boiling and thin-film evaporation heat transfer coefficients. Conversely, hydrophobic structured surfaces, i.e. superhydrophobic surfaces have recently shown a promise to push the limits of condensation heat transfer. In addition to these fundamental studies, we are also investigating avenues to actively manipulate droplet morphology and wetting states to design microfluidics devices for practical applications. For example, we developed dynamically tunable micropillar arrays, where the tilt angle is controlled by an external magnetic field. The tunable surface design promises great potential for thermal management, microfluidics, biological and optical applications.

The wettability and microfluidics initiatives at the Device Research Laboratory are supported by a number of agencies including National Science Foundation, Office of Naval Research, Air Force Office of Scientific Research, and Battelle National Security Global Business.

Boiling -- Boiling is a very effective mode of heat transfer and is used in a multitude of applications from day-to-day cooking to industrial scale thermal management and power generation. Despite the ubiquitous presence of boiling, the phenomenon is not completely understood mainly due to the complex phase change process, liquid-vapor wetting dynamics, convective flows, and temperature fluctuations involved. Furthermore, there exists a growing demand for the capability of transferring heat at higher rates as well as for improving device efficiencies.

At the Device Research Laboratory, we have experimentally demonstrated that, by introducing superhydrophilic surfaces with micro-and nanostructures into pool boiling systems, the maximum heat flux of boiling can be increased before catastrophic dryout occurs. We have also investigated and explained the underlying mechanism of delayed dryout, which is attributed to the enhanced liquid transport on superhydrophilic structures aided by capillary pressure. In continuation of this effort, we have also implemented microstructured superhydrophilic surfaces in flow boiling systems where heat transfer performance are also influenced by many other factors such as flow rate, channel geometry, and subcooling. We are currently working to better understand the role of microstructures in flow boiling in order to develop better thermal management strategies. In addition to structured surfaces, we are investigating how surfactant additives that adsorb to solid-liquid and liquid-vapor interfaces can enhance boiling performance by increasing bubble nucleation.

Boiling work at the Device Research Laboratory is funded by the Masdar Institute and the Battelle Memorial Institute.

Evaporation -- Evaporation is an effective cooling method widely utilized in nature (e.g., transpiration and perspiration) as well as for the thermal management of electronic devices. It also plays a significant role in water desalination, humidification, and steam generation. Previous studies that attempted to characterize the heat and mass transfer at the interface level were generally limited to low interfacial heat fluxes (&st; 100 W/cm2), while it is desirable to have high heat fluxes in many applications.

Experimentally, it has been challenging to characterize high flux interfacial transport. First, the temperature of the liquid-vapor interface needs to be measured in an accurate and noninvasive way. Second, it is necessary to minimize the transport resistance associated with the heat supply, liquid refilling, and vapor removal. Otherwise, it is not possible to reach a high flux across the interface. Third, the evaporation rate is very sensitive to contamination in the system. If the contaminants do not evaporate, the liquid- vapor interface accumulates the contaminants which eventually clogs the pores. We developed an ultrathin, nanoporous membrane evaporator with a membrane thickness t = 200 nm and pore radius r = 65 nm. With this device, we were able to address all of the above challenges to experimentally investigate evaporation in the high flux regime. The evaporation region of the device is 0.26 mm x 3.4 mm in area, coated with Au which serves as both a resistive temperature detector (RTD) and a heater. This configuration allows us to measure the temperature very accurately, close to the interface since the distance between the RTD and the interface is at most r. It also minimizes the heat conduction resistance in the liquid phase (which scales with r) as well as the flow resistance along the pore (which scales with t) and mitigates the contamination risk. We show that it is possible to reach high interfacial heat fluxes (500 W/cm2) with pure evaporation into the air ambient. We also demonstrate that by removing the vapor diffusion resistance with a pure vapor ambient, the interfacial transport can be further enhanced. Overall, this ultrathin nanoporous membrane evaporator facilitates the fundamental understanding of the interfacial transport and we are working on further utilization of high flux evaporation in desalination, steam generation, and thermal management.

The evaporation research is funded by the Air Force Office of Scientific Research with Dr. Ali Sayir as the program manager.

Condensation -- Condensation is a phase change phenomenon often encountered in nature but also harnessed for industrial applications including power generation, thermal management, desalination, and environmental control. For the past eight decades, researchers have focused on creating surfaces which allow condensed droplets to be easily removed by gravity for enhanced heat transfer performance. Recent advancements in nanofabrication have enabled increased control of surface structuring for the development of superhydrophobic surfaces and lubricant infused surfaces with even higher droplet mobility.

At the Device Research Laboratory, we theoretically and experimentally study superhydrophobic surfaces to enhance condensation heat transfer for water. We work on identifying challenges and new opportunities to advance these surfaces for broad implementation into thermo-fluidic systems. For example, the recent discovery of jumping droplet electrostatic charging has led to applications in condensation heat transfer enhancement and energy harvesting. We also experimentally study lubricant infused surfaces to enhance condensation for water and low surface tension fluids. More recently, we have developed a design guideline for lubricant infused surfaces.

In addition, we study the fabrication, characterization, wettability, and interfacial dynamics of superhydrophobic materials during condensation to examine the role of surface structure on emerging droplet morphology, nucleation density, droplet growth rate, and departure characteristics. Furthermore, we seek to develop scalable fabrication techniques for creating hydrophobic surfaces with experimentally demonstrated heat transfer enhancement, and we investigate the robustness of these surfaces under industrial conditions as well as the effects on contamination on wetting properties.