Mahamudur Rahman

Role of wickability on the critical heat flux of structured superhydrophilic surfaces.

While superhydrophilic coatings with enhanced wetting properties have been shown to increase the pool boiling critical heat flux (CHF), the role of nanostructures on its enhancement is not clear. Here, biological templates have been used to demonstrate that wickability is the single factor dictating CHF on structured superhydrophilic surfaces. The flexibility of biotemplating using the Tobacco mosaic virus has been leveraged to create surfaces with varying scales, morphologies, and roughness factors. Their wickabilities have been quantified via the wicked volume flux, a phenomenological parameter analogous to the contact angle, and the role of wickability on CHF has been demonstrated using data from over three dozen individual surfaces. These results are repeatable and independent of the substrate material, surface fouling, structure material, morphology, and contact angle as well as the structure scale. An experimentally validated correlation for CHF has been reported on the basis of the dimensionless wickability. Additionally, the surfaces have achieved a CHF of 257 W/cm(2) for water, representing the highest reported value to date for superhydrophilic surfaces. While the role of wickability on CHF has often been cited anecdotally, this work provides a quantitative measure of the phenomena and provides a framework for designing and optimizing coatings for further enhancement.

Increasing Boiling Heat Transfer using Low Conductivity Materials.

We report the counterintuitive mechanism of increasing boiling heat transfer by incorporating low-conductivity materials at the interface between the surface and fluid. By embedding an array of non-conductive lines into a high-conductivity substrate, in-plane variations in the local surface temperature are created. During boiling the surface temperature varies spatially across the substrate, alternating between high and low values, and promotes the organization of distinct liquid and vapor flows. By systematically tuning the peak-to-peak wavelength of this spatial temperature variation, a resonance-like effect is seen at a value equal to the capillary length of the fluid. Replacing ~18% of the surface with a non-conductive epoxy results in a greater than 5x increase in heat transfer rate at a given superheat temperature. This drastic and counterintuitive increase is shown to be due to optimized bubble dynamics, where ordered pathways allow for efficient removal of vapor and the return of replenishing liquid. The use of engineered thermal gradients represents a potentially disruptive approach to create high-efficiency and high-heat-flux boiling surfaces which are naturally insensitive to fouling and degradation as compared to other approaches.

Full-Field Dynamic Characterization of Superhydrophobic Condensation on Biotemplated Nanostructured Surfaces

While superhydrophobic nanostructured surfaces have been shown to promote condensation heat transfer, the successful implementation of these coatings relies on the development of scalable manufacturing strategies as well as continued research into the fundamental physical mechanisms of enhancement. This work demonstrates the fabrication and characterization of superhydrophobic coatings using a simple scalable nanofabrication technique based on self-assembly of the Tobacco mosaic virus (TMV) combined with initiated chemical vapor deposition. TMV biotemplating is compatible with a wide range of surface materials and applicable over large areas and complex geometries without the use of any power or heat. The virus-structured coatings fabricated here are macroscopically superhydrophobic (contact angle >170°) and have been characterized using environmental electron scanning microscopy showing sustained and robust coalescence-induced ejection of condensate droplets. Additionally, full-field dynamic characterization of these surfaces during condensation in the presence of noncondensable gases is reported. This technique uses optical microscopy combined with image processing algorithms to track the wetting and growth dynamics of 100s to 1000s of microscale condensate droplets simultaneously. Using this approach, over 3 million independent measurements of droplet size have been used to characterize global heat transfer performance as a function of nucleation site density, coalescence length, and the apparent wetted surface area during dynamic loading. Additionally, the history and behavior of individual nucleation sites, including coalescence events, has been characterized. This work elucidates the nature of superhydrophobic condensation and its enhancement, including the role of nucleation site density during transient operation.

Boiling Enhancement on Nanostructured Surfaces with Engineered Variations in Wettability and Thermal Conductivity

ABSTRACT This work provides fundamental insights into the underlying mechanisms of pool boiling enhancement using a variety of different engineered surface designs. Specifically, the effects of nanostructured coatings, surfaces with mixed wettability, and surfaces with in-plane variations in thermal conductivity are investigated. The positive and negative impacts of each design on the onset of nucleate boiling, heat transfer coefficient, bubble dynamics and the ebullition cycle, as well as critical heat flux have been characterized. It is seen that while several techniques enhance one element of the boiling process, they can degrade others. This analysis has led to the design, fabrication, and characterization of complex heterogeneous surfaces by combining multiple engineered surface techniques. Nanostructured surfaces with variations in substrate thermal conductivity have been shown to increase critical heat flux by a factor of 2.6× as compared to bare copper substrates. In addition, nanostructured surfaces with engineered variations in substrate conductivity as well as surface wettability have been shown to increase heat transfer coefficient by more than a factor of 10×.

Experimental Characterization of Inward Freezing and Melting of Additive-Enhanced Phase-Change Materials Within Millimeter-Scale Cylindrical Enclosures

The inward melting and solidification of phase-change materials (PCM) within millimeter-scale cylindrical enclosures have been experimentally characterized in this work. The effects of cylinder size, thermal loading, and concentration of high-conductivity additives were investigated under constant temperature boundary conditions. Using a custom-built apparatus with fast response, freezing and melting have been measured for time periods as short as 15 s and 33 s, respectively. The enhancement of PCM thermal conductivity using exfoliated graphene nanoplatelets (xGnPs) has also been measured, showing a greater than 3× increase for a concentration of 6 wt.%. Reductions in the total melting and freezing times of up to 66% and 55%, respectively, have been achieved using xGnP concentrations of only 4.5 wt.%. It is shown that the phase-change dynamics of pure and enhanced PCM are well predicted using a simple conduction-only model, demonstrating the validity of approximating enhanced PCM with low additive loadings as homogenous materials with isotropic properties. While general consistency between the measurements and model is seen, the effect of additives on heat transfer rate during the initial stages of freezing and melting is lower than expected, particularly for the smaller cylinder sizes of 6 mm. These results suggest that the thermal resistance of the PCM is not the limiting factor dictating the speed of the solid–liquid interface during these initial stages.