Droplet Surface Interactions for Hotspot Cooling

Achievement date: 
2016
Outcome/accomplishment: 

Researchers at the NSF-funded Engineering Research Center (ERC) for Power Optimization for Electro-Thermal Systems (POETS), headquartered at the University of Illinois, have investigated how the stiffness of an elastic surface affects the bouncing-versus-splashing behavior of droplets on such surfaces. Understanding that behavior is important because it affects heat transfer between the droplets and the surface. 

Impact/benefits: 

This work offers insights into new avenues for improving the performance of spray- cooling technologies for hotspot cooling. In particular, the researchers showed that substrate flexibility can lead to a substantial reduction in contact time compared to equivalent rigid surfaces.

When integrated into heat transfer equipment and tailored for specific cooling applications, surfaces of various elasticities could enable tunable heat transfer based on the temperature-dependent stiffness of the substrate. Such surfaces could be built into thermal management vapor chambers to enhance their effectiveness and hence increase overall system power density.

 

Explanation/Background: 

Through high-speed imaging studies, Professors Nenad Miljkovic and William King, along with their graduate student Patty Weisensee, analyzed the droplet dynamics of millimeter-sized droplets impinging on elastic substrates of different stiffness (very soft to rigid).

Results illustrated in the figure show that contact times on the flexible substrates can be reduced by a factor of 2 compared to rigid surfaces, while the spreading time and maximum spreading diameter remain unchanged. Upon impact, the droplet excites the substrate to oscillate. By tailoring the stiffness and natural frequency of the substrate, the upward motion of the substrate oscillation can cause the droplet to lift off in a pancake-like shape at contact times shorter than on a rigid surface, analogous to jumping on a trampoline.

Based on a transient droplet-impact heat transfer model, the researchers showed that for single-droplet and multiple-droplet impacts the total heat transfer is minimized and maximized, respectively, for small droplets (~1 mm) and those with high impact velocities (~2 m/s).