Chemical Engineering Seminar
During the unfamiliar isolation at the start of the COVID pandemic, the baking of sourdough bread became fashionable again. People were newly introduced to a very old problem. Bread dough is a complex soft material that integrates water, fat, sheets of aggregated proteins and bubbles of air and carbon dioxide to achieve a particular texture. It can seem "sticky," "stretchy," "tough," "tender," or "airy," among other sensorial descriptors. Those sensations may change depending on whether one kneads the dough quickly in a mixer or hard with a pair of hands. These same sensations also evolve over time as the structure of the dough changes both physically and chemically. While home bakers rely on millennia of human experimentation to guide them through the production of sourdough, laboratory scientists and practicing engineers use techniques from the field of rheology to quantify similar "sensations" when designing new kinds soft materials for applications in our modern world.
One challenge in the field of rheology and soft materials design is understanding, and ultimately predicting, how materials deform in response to different magnitude stresses applied over different time scales. Bread dough left on the counter might slump smoothly, but bread dough tugged on suddenly might appear stiff. If the sudden tug is hard enough the dough might even fracture. It is quite challenging to characterize time-dependent and load-dependent responses generically. Physically, one wants to understand how the stress that develops in a material depends on that material's deformation history. In this talk, I will discuss some new methods of rheology developed in my lab at MIT that enable characterization of this relationship between stress and deformation history more robustly. A key insight that I will explore is that this functional relationship between stress and deformation history can written in terms of a kind of power series expansion in deformation history that is properly named a Volterra series. The "coefficients" of this Volterra series expansion are material properties, and some of them can easily be measured with cleverly crafted experiments on commercial rheometers. These new, nonlinear mechanical properties are the "hidden dimensions" of soft materials.
One common mechanical testing experiment uses a rheometer to induce a sinusoidal deformation in a soft material and to measure the time-dependent stress required to drive that oscillation. Rather than drive the rheometer with a single "tone," one could induce a deformation with a superposition of three distinct tones. This is like playing a chord on the rheometer. The overtones observed in the stress required to induce this three-tone deformation can be used to directly measure some of the Volterra series coefficients of the soft material being tested. In addition to introducing this new rheological technique, I will show how data from such experiments can be used to learn about the micromechanical details of some complex fluids. I will also discuss several potential applications for these new nonlinear mechanical descriptors of soft materials.