2/19/2015 Julia Cation
Written by Julia Cation
New MechSE assistant professor Yuhang Hu studies pitcher plants—carnivorous plants with modified leaves that form a deep, prey-trapping cavity—to analyze certain bio-inspired systems and soft materials. The phenomenon with these particular plants is that their surface becomes very slippery when wet, causing ants, for instance, to slide down inside and get digested. However, the pitcher plant’s surface is slippery only occasionally. When dry, that same surface acts as a sort of adhesive, allowing insects to walk easily on it and access the nectar from the plant.
This varying surface property is what led to Hu’s development of a new material system with optical properties and wettability that can be continuously tuned by mechanical stimuli.
“We wanted to know if we could make a surface like the pitcher plant that is both slippery and non-slippery, with active control. From my basic training in solid mechanics and from my previous research on poroelastic properties of hydrogel, which is also a type of solid-liquid composite soft material, I knew that if the material is stretched, it will change the pore pressure inside, causing the liquid to flow. The mechanical stimulus of stretching turned out to be the other half of the story,” she said.
Based on this central idea, she and other researchers successfully developed a synthetic material that has continuously adjustable characteristics.
“Such a material is made of a liquid film supported by a nanoporous elastic substrate. As the substrate deforms, the liquid flows within the pores, causing the smooth and defect-free surface to roughen through a continuous range of topographies. We show that a graded mechanical stimulus can be directly translated into finely tuned, dynamic adjustments of optical transparency and wettability. In particular, we demonstrate simultaneous control of the film’s transparency and its ability to continuously manipulate various low-surface-tension droplets from freesliding to pinned. This strategy should make possible the rational design of tunable, multifunctional adaptive materials for a broad range of applications,” said Hu.
In addition to pitcher plants, Hu analyzes other biological systems that could potentially be used to create new materials and devices with multifunctionalities. She is interested in materials or systems composed of soft solid and liquid, and she studies the fundamental mechanics behind the nature systems that affect adaptability and efficiency. She also develops robust mechanical testing techniques to characterize these delicate materials.
Hu said she sees a lot of potential in her work for future study in biology and bioengineering—on one hand, utilizing the bio-inspired material and device to tailor the morphology of cells and tissues, while, on the other hand, applying the mechanical testing technique for softer materials that she developed during her PhD work to characterize engineered biological materials.
“It is going to be a new adventure for me, but as my name suggests—Yuhang means ‘exploring the outer space’ in Chinese—I always like to explore the unknowns and challenge myself,” she said.
Hu earned her bachelor’s degree in engineering mechanics from Shanghai Jiao Tong University in China in 2005; a master’s degree in civil and environmental engineering from Nanyang Technological University in Singapore in 2007; an MS in applied physics from Harvard in 2009; and a PhD in solid mechanics from Harvard in 2011. She completed her postdoctoral work in Harvard’s biomimetic lab.
Hu joined the MechSE department in January and is currently teaching ME 532 (Fracture Resistant Design), preparing her lab, and mentoring the students in her research group.