Virginia Tech Mechanical Engineering Annual Report 2017 Annual Report | Page 25

Looking to nature for self-cleaning surfaces Jiangtao Cheng “In nature, lotus leaves can achieve self-cleaning by main- taining dew conden- sate in droplet form, which can easily roll off the leaf surface. In industry, biomimetic lotus-leaf-like surfaces can not only allow for easy droplet remov- al at micrometric length scales during condensation but also promise to enhance heat transfer performance if designed properly. In this paper Dr. Cheng’s group report energy-based analysis of growth dynamics of dropwise con- densates on biomimetic surfaces with two-tier mi- cro/nano-textures, which are superior to solely nanotextured surfaces in controlling nucleation density. To understand the role of condensate state transition, i.e., from partially wetting state (PW) to suspended Cas- sie state (S), in enhancing condensation heat transfer, they considered adhesion energy, vis- cous dissipation and contact line dissipation as the main portion of resistant energy that needs to be overcome by the condensate drop- lets formed in surface cavities. By minimizing the energy barrier of the state transition, they optimized first tier roughness on the hierar- chically textured surfaces allowing condensates to grow preferentially in the out-of-plane direction. The nanoroughness of the second tier plays an important role in abat- ing the adhesion energy in the cavities and contact line pinning. From the perspective of molecular kinetic theory (MKT), the hierar- chically engineered surface is bene- ficial to remarkably mitigating contact line dissipation. This study indicates that scaling down surface roughness to submicron scale can facilitate self-propelled condensate removal.” When Thermodynamics Meets Quantum Mechanics Micha el von Spakovsky It has long been known that while mechanics is funda- mental in its description, thermodynamics is phenom- enological. However, things have changed dramatically in that the gap between these two disciplines has consis- tently been bridged with the help of quantum mechanics. A widespread view that thermodynamics is already complete as evidenced by the 1974 remark: “[w]e may be reasonably sure that a treatise on, say, ther- modynamics, published in the year 2000 will not be fundamentally different from one available today …” has been proven wrong due to significant conceptual breakthroughs in thermodynamics resulting in part from the miniaturization of devices at the quantum level. Differing from other quantum-level thermody- namic methodologies, the “steepest-entropy-ascent quantum thermodynamics” (SEAQT) framework, which our Center for Energy Systems Research has been helping develop and apply, has attracted considerable attention asserting that the second law of thermodynamics supplements rather than emerges from quantum mechanics. SEAQT has developed over decades, growing ever more rapidly and addressing fundamental issues such as a consistent description of heat and work as fun- damental tools at the quantum level. In this research, this issue and others have been and are continuing to be addressed, making a consistent description of quantum/nano-scale thermal machines possible and shedding new light on various fundamental but yet unresolved questions of thermodynamics. This in turn has led to a unified understanding of multi-scale phenomena. From a practical standpoint, this methodology can now be and has already been applied across all spatial and temporal scales, i.e., from the atomistic to the macroscopic, doing so with a single kinematic and dynamic description and without the typical limiting assumptions that come with traditional or classical approaches. We have furthermore rigorously gener- alized the equilibrium description of classical ther- modynamics and the near-equilibrium realm to the entire non-equilibrium region.