Matthew McCarthy’s research while affiliated with Drexel University and other places

This work demonstrates the development of a novel backside thermography technique based on the temperature sensitivity of laser-induced fluorescence in flowing two-dye solutions. The approach utilizes visible light and optically transparent packaging materials to obtain spatially resolved transient thermal measurements. This makes it a relatively simple, inexpensive, and flexible approach for lab-scale experimental characterizations using economical cameras and optics. Additionally, both heating and cooling of the thermography stage can be achieved by passing temperature-controlled water through plastic packaging components. A custom-built experimental setup was designed, constructed, and used to study the performance of seven two-dye Rhodamine B (RhB)-Rhodamine 110 (Rh110) fluorescent solutions. The effect of dye concentration ratio on sensitivity, maximum frame rate, and excitation area was characterized. Using an optimal dye concentration of unity, it was shown that frame rates of 265–3200 Hz are achievable for excitation areas with diameters of 15.5–4.3 mm, respectively, using the current setup. The calibrated system was used to demonstrate in-situ measurements showing the importance of two-dye light compensation, as well as backside thermography using a simple droplet contact method to investigate temporal response. Experiments were conducted in the range of 25–55 °C with a spatial resolution of 30 µm. The experimental uncertainty of the temperature measurements was calculated to be ±2.1 °C and the technique's ability to account for error due to light fluctuations and non-uniform illumination was demonstrated. Finally, the effect of dye photobleaching during prolonged testing was studied and it was shown that photobleaching can be reduced, and even eliminated, by maintaining a nominal low flow rate of the pumped two-dye solution.

We present a microamphiphilic surface to promote the formation of a thin, stable liquid film during condensation. The surface consists of a hydrophilic micropillar array with hydrophobic pillar tips and was made using photolithography, deep reactive ion etching, and liftoff. The hydrophobic tips prevent the liquid film from growing thick, thereby keeping the thermal resistance low without the cyclical growth and shedding process of dropwise condensation. The wetting behavior was modeled analytically, and the parameters required for film formation were determined and verified with ESEM experiments. When a surface filled with condensate and lacked a low-pressure zone for the water to leave, a rupture event occurred, and a large Wenzel droplet emerged to flood the surface irreversibly. A number of strategies were found to combat rupture events. Tilting the surface vertically and dipping in a liquid pool gave the condensate a low-pressure region and prevented rupture. Irreversible flooding can also be avoided by ensuring that the emerged droplet was a nonwetting, highly mobile Cassie droplet. Parameters for Cassie-stable amphiphilic surfaces were determined analytically, but the high aspect ratios required prevented the manufacture of these surfaces for this study. Instead a hierarchical design was presented that demonstrated emerged Cassie droplets without challenging the manufacturing limits of the microfabrication procedure. This design avoided Wenzel droplet flooding without the need for a designated low-pressure zone. Additionally, sites for Cassie emergence could be engineered by removing a single pillar from the array at a designated location.

The use of micro/nano-scale structures has been shown to enhance critical heat flux (CHF) during pool boiling in recent studies. A correlation between wicking rate and CHF enhancement for structured superhydrophilic surfaces has been reported in prior work of the authors. In that work, a non-dimensional correlation was developed and validated using only water as the working fluid. In this study, a highly wetting fluid (FC-72) was used to demonstrate the applicability of this correlation on structured surfaces for non-aqueous liquids. This has been achieved using a simple modification of the experimental procedure for highly-wetting fluids. This experimental modification shows no effect on the quantification of liquid wicking rate. Numerous structured superhydrophilic surfaces have been fabricated ant tested, including micro and nano-scale structures, as well as hierarchical surfaces which showed the highest CHF enhancement (200%). More importantly this work demonstrates the validity of the nondimensional parameters used in the proposed CHF correlation and its overall applicability to a wide range of non-aqueous liquids.

Condensation heat transfer can be altered significantly by changing the texture and material of a surface to promote droplet removal and therefore lower thermal resistance. These designs are often expensive and fragile, however, and are fabricated using micro- or nano-scale features that are not easily implemented in real-world systems. Here, we present a novel macro-machined amphiphilic surface that promotes droplet removal and resists permanent flooding via a spontaneous dewetting transition. While much of the research in condensation involves condensing on surfaces that are fully or mostly hydrophobic, droplets on the surface presented here nucleate and grow inside the structure on a hydrophilic material. The absence of any coating between the liquid and the conductive surface has the benefits of both decreasing thermal resistance and enhancing nucleation density. When the liquid grows to a critical size inside the channel, its elongated shape becomes unstable and spontaneously dewets to form rounded droplets on the hydrophobic fin peaks. The removal of liquid from the channels promotes new growth on bare hydrophilic material while the emerged rounded droplets can more easily shed from the hydrophobic fins. The dewetting phenomenon is shown experimentally and characterized analytically such that a desired critical water slug length could be designed by changing geometric parameters of the surface structure. The macro-scale machined surface is also more durable than typical nanofabricated surfaces and easier to manufacture, making the surface more applicable to use in real-world systems. Spontaneous condensate dewetting on amphiphilic structure is expected to enhance the study of inhibiting flooding on condensing surfaces and provide new pathways for droplet shedding techniques without a requirement for nano-thin hydrophobic coatings.

High porosity nanostructured coatings have been extensively studied for their use in enhancing liquid-to-vapor phase change due to their ability to wick liquids laterally across surfaces during boiling. While the effects of these coatings on the maximum heat transfer rate achievable (the critical heat flux) is now well understood, the impact on boiling efficiency (the heat transfer coefficient) is less clear. In this work a novel experimental apparatus is used to take heat transfer measurements beneath growing and departing bubbles on nanostructured surfaces. By independently tuning the surface heat flux and bubble departure time, IR thermography is used to directly visualize and characterize surface superheat, heat flux, and heat transfer coefficient during the highly transient bubble ebullition cycle. It is shown that while flat surfaces exhibit large spatial and temporal variations in surface temperature and heat flux, the nanostructured coatings produce a uniform temperature profile with enhanced heat transfer due to evaporation from the nanostructure-supported liquid films beneath the bubble. This work demonstrates the relative importance of advancing and receding contact lines, as well as the quenching process, on the overall thermal performance of structured and non-structured surfaces. It is seen that the combined effects produce a net increase in heat transfer coefficient of over 30%, averaged over the entire ebullition cycle and throughout the entire area of influence. Additionally, the impact of viscous resistance and the importance of nanostructure dry out has been studied by tuning the ebullition cycle time to create dry spots. This work shows for the first time the role of nanostructured coatings and thin-film evaporation during nucleate boiling, and it provides a framework to understand the complicated nature of nanostructured boiling across all portions of the boiling curve from nucleation to critical heat flux.

Surface wettability plays an important role in dew collection. Nucleation is faster on hydrophilic surfaces, while droplets slide more readily on hydrophobic surfaces. Plants and animals in coastal desert environments appear to overcome this tradeoff through biphilic surfaces with patterned wettability. In this study, we investigate the effects of millimeter-scale wettability patterns, mimicking those of the Stenocara beetle, on the rate of water collection from humid air. The rate of water collection per unit area is measured as a function of subcooling (ΔT = 1, 7, and 27°C) and angle of inclination (from 10° to 90°). It is then compared for superbiphilic, hydrophilic, hydrophobic, and surperhydrophobic surfaces. For large subcooling, neither wettability nor tilt angle has a significant effect because the rate of condensation is so great. For 1°C subcooling and large angles, hydrophilic surfaces perform best because condensation is the rate-limiting step. For low angles of inclination, superhydrophobic samples are best because droplet sliding is the rate-limiting step. Superbiphilic surfaces, in contrast to their superior fog collecting capabilities, generally collected dew at the slowest rate due to their inherent contact angle hysteresis. Theoretical considerations suggest that this finding may apply more generally to surfaces with patterned wettability.

This work characterizes the inward melting and freezing of phase change materials (PCMs) within small diameter polymer tubes under convective boundary conditions. Using air as the heat transfer fluid, a custom-built experimental apparatus was used to control the convective heating and cooling of vertically oriented PCM-filled tubes with an outer diameter of 3.175 mm. Various air speeds and air temperatures were investigated, as well as the effects of wall thickness and material properties using three different polymer encapsulant tubes. Volume fraction, latent heat transfer rate, and the associated overall heat transfer coefficient have been measured experimentally during the transient melting and freezing processes. Using a commercially available paraffin-based PCM with a nominal melting temperature of 35 °C, phase change times ranging from 50 s to 180 s were recorded with air speeds and air temperatures of 4.7 – 15.3 m/s and 25.3 – 62.6 °C, respectively. Overall latent heat transfer coefficients as high as 200 W/m²K were measured and shown to decay with time as the PCM changes phase during inward melting and freezing. For the thinnest encapsulants tested here, the results were consistent with a simplified one-dimensional quasi-steady-state thermal circuit model. Deviations from this model were observed and explained, including the importance of the wall thickness and thermal conductivity of the encapsulant.