Dissertations and Theses

Date of Award


Document Type



Chemical Engineering

First Advisor

Xi Chen


Material science, Water-responsive materials, Evaporation energy harvesting


Biological systems have developed remarkable water-responsive (WR) structures that deform in response to changes in relative humidity (RH) and convert evaporation energy (chemical potential of water) to mechanical energy. For example, the scales of pinecones open and release pine seeds when the environment is dry. Prior research has shown that the spores of Bacillus (B.) subtilis exhibit a high WR actuation energy density of 21.3 MJ m-3, illustrating the great potential of using WR materials to develop high-performance actuators for various energy-related applications. However, the fundamental WR mechanisms remain poorly understood, which impedes optimization and real-life application of WR materials. This thesis presents that a supramolecular component of peptidoglycan (PG) dominates spores’ water-responsiveness. When responding to RH changes, PG shows fast WR speed (~0.1 s), and its WR energy and power densities and energy conversion efficiency reach 72.6 MJ m-3, 9.1 MW m-3 and 66.8%, respectively, orders of magnitude higher than those of frequently used actuator materials. PG’s extraordinary WR performance could be attributed to the super-viscous nanopore water (viscosity reaches ~16.4 Pa·s) and PG's stiff, deformable supramolecular structures. To further investigate the role that nanopore water’s properties and water-structure interactions play in PG’s water-responsiveness, we mimicked the PG’s structures and developed nanometer-scale channels, whose vertical sidewalls can bend in response to RH changes. Using these nanochannels, we discovered that, during a dehydration process, the nanopore water withstands tension, which bends the channel walls and leads to elastic energy storage. In addition, we have also demonstrated strategies of using WR materials to actuate engineering structures as well as new kinds of evaporation engines that can continuously capture energy from evaporation and convert that energy into mechanical motion. Our work could provide pioneering methods to harness the untapped energy source of natural evaporation.

Available for download on Saturday, August 21, 2027