Boiling heat transfer plays a key role in a wide range of terrestrial applications, including power plants that produce the majority of electricity in the US, heating and cooling of buildings, desalination and distillation. The boiling heat transfer performance is directly related to the removal rate of bubbles generated during boiling. However, in most terrestrial applications, bubble departure is naturally driven by buoyancy; it is difficult to control the bubble departure size and frequency. A simple method that can actively control bubble motion with large tuning range is highly desirable, since it would significantly expand the range of achievable boiling heat transfer rate, with the potential to improve the efficiency of power plants and reduce energy consumption in building thermal management. This project aims to develop a new method to control bubbles and droplets, by exploiting liquids whose surface tension can be changed with light. The microgravity environment eliminates the interference of buoyancy, which allows us to purely observe and understand this light-driven fluid motion. This method can be generalized to manipulate multi-phase fluid for condensation processes and applications including precision control in 3D printing, lab-on-a-chip microfluidics for biomedical and optical applications.
The overarching goal of this research is to achieve dynamic manipulation of multi-phase fluid motion using light and photo-responsive surfactants, and apply it to enhance boiling heat transfer. Photo-responsive surfactants can reversibly switch their molecular conformation when illuminated with light, resulting in a dynamically tunable and spatially reconfigurable surface tension that can drive multi-phase fluid motion (the photo-Marangoni effect). The project tasks include: (i) experimentally test the depinning criteria and migration velocity of droplets and bubbles controlled by light, (ii) develop the first modeling framework for the photo-Marangoni effect, and ultimately (iii) promote bubble departures during boiling to enhance thermal transport by optically ?pinching off? bubbles with control. The use of microgravity is essential to enable large length scales exceeding the typical capillary lengths on earth, as well as long time scales for bubble/droplet departure, which greatly reduces the requirements of microscopic and high-speed visualization. Microgravity will also allow direct experimental observation of the proposed light-controlled motion without buoyancy and natural convection, which ensures accurate fundamental understanding and comparison to theory. This light-tuning method will serve as an effective yet simple new platform for dynamic fluid and heat transfer manipulation. This platform will significantly contribute to developing new research capabilities and inspiring new applications beyond heat transfer, such as novel approaches to droplet-based biochemical assays, dynamic patterning and manufacturing.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
