IMX Colloquium - Surface engineering for phase change heat and mass transfer: controlling nucleation and bubble dynamics

Boiling is one of the most efficient modes of heat transfer, underlying many energy-intensive processes. Advancements in surface engineering have enabled significant enhancement of boiling heat transfer. However, performance gains rarely transfer across conditions or geometries. Two challenges illustrate why: bubble nucleation behavior is poorly reproducible; and increasing nucleating density to raise the heat transfer coefficient typically lowers the system’s maximum achievable heat flux. This talk presents our efforts in addressing each challenge by identifying the governing mechanism and designing for it.
In the first part, micro-engineered silicon surfaces isolate two governing length scales: the hydrodynamic boundary layer thickness, which controls nucleation stability through inter-site shielding, and the bubble departure diameter, which governs vapor removal after full activation. We experimentally show that nucleation stability responds to the former length scale (~0.24 mm); heat transfer in the activated regime responds to the latter length scale (~2.5 mm).
The second study addresses the high-heat-flux limit, where large vapor structures block liquid return. Capillary wicking through copper inverse opals (~10 μm pores) supplies liquid beneath the vapor. Crucially, the pore cavities are themselves the nucleation sites, so wicking capacity and nucleation density derive from the same structure. Further, CuO nanograss on the pore walls suppresses vapor penetration into the porous layer, enabling thicker structures and greater capillary head. The result is simultaneous enhancement of heat transfer coefficients and maximum heat fluxes.
Together, the two works show that mechanism-targeted surface design yields gains that empirical topography optimization does not. They point toward a broader design principle: that nucleation and interfacial transport, treated as controllable rather than given, offer a promising path to engineering phase change processes across applications.

https://dx.doi.org/10.1021/acsami.9b20520?ref=pdf
https://doi.org/10.1038/s41467-019-10209-w 

Bio: Zhengmao Lu is a Tenure Track Assistant Professor of Mechanical Engineering at EPFL. Prior to joining EPFL, Zhengmao was a postdoctoral scholar in the Department of Materials Science and Engineering at MIT with Prof. Jeffrey Grossman. He received his Ph.D. and M.S. in Mechanical Engineering from MIT, both advised by Prof. Evelyn Wang. At EPFL, Zhengmao leads the Energy Transport Advances Laboratory (η-Lab), aiming to deepen our understanding of phase change phenomena and develop more sustainable energy and water technologies by optimizing interfacial transport. Zhengmao is a recipient of the ERC Starting Grant, MicroFIPS Outstanding Early Career Award,  Keck Travel Award in Thermal Sciences, and the Outstanding Graduate Research Award from MIT Mechanical Engineering.