Douglas Holmes Explores the Science of Swelling
By Sara Cody
A murmur of thunder and pulses of lightning roll across the night sky, illuminating the formidable outline of a forest of baobab trees, an ancient species found in Africa. The soothing voice of British naturalist David Attenborough narrates as the flowers of the baobab begin to blossom as they fill with water, unfurling the delicate tissue in less than a minute as the water guided its new shape. This clip from BBC’s Planet Earth inspired Assistant Professor Douglas Holmes (ME, MSE), who runs the Mechanics of Slender Structures (MOSS) research group, to pursue an experiment that was recently featured on the cover of Soft Matter.
“My research group does a lot of work with soft matter physics, so I was really excited when I saw these baobab flowers,” says Holmes. “I was curious if the dramatic shape change causing the flowers to open was caused by swelling, so I wanted to see if I could recreate that in a laboratory setting. Engineers are constantly looking for new ways to make materials change shape on command, and I thought this phenomenon might provide insight into utilizing the effects of swelling in a controlled way.”
Holmes devised a simple experiment where he took two thin sheets of rubber, hung them from a rod and lowered them into a bath of fluid until their tips were submerged. Initially, the sheets pulled together, but as the liquid permeated the structure, the swelling caused the ends to curl up and peel away from each other in an outward motion, similar to the movement of the baobab blossoms.
“It took some trial-and-error to get the materials for the experiment just right, because we needed to find a balance where the wetting, swelling and bending were all on the same playing field and competing with each other,” says Holmes. “Depending on which of these forces we focus on, this sweet spot we found acts as a good baseline. Even a simple experiment like this can lead to a variety of complicated physics.”
During the experiment, as liquid permeated the rubber sheets, elastocapillary swelling caused them to curl. Video provided by Assistant Professor Douglas Holmes (ME)
By understanding the physics of how and why this phenomenon, known as elastocapillary swelling, happens, Holmes can apply his engineering perspective to figure out a way to develop materials from thin structures that are able to change shape on command, which has a host of potential applications.
“There are a variety of different stimuli you can use to change an object’s shape, like heat, voltage, light, or fluid. In our case, we are using swelling,” says Holmes. “By incorporating fluid in the structure to begin with, we could control the swelling to make the material change shape, like the baobab flower. Imagine a smart needle that can be injected into something, and using this concept, you can tell it to bend back and forth while navigating towards a certain target.”
Holmes also sees potential application in industry, where companies that manufacture soft materials would benefit from the knowledge of how to account for environmental factors like humidity, which could cause swelling that would put stress on the manufactured materials. Holmes has even been contacted by cosmetics companies which are interested in developing hair products that prevent frizzing.
“This study was an interface between things that are soft and things that are thin, so with any small change to the environment, you are bound to see a drastic deformation in the shape,” says Holmes. “Going forward, we’ll continue to explore how to get that object to bend, and how different surroundings and stimuli affect it.”