Structure & Dynamics of Creep Deformation in Disordered Materials

Non-technical Abstract All around us are materials that appear to be solid, but that actually “creep” – they flow in a very slow and mysterious manner. The “glacial pace” of flowing ice is an example of creep. Creep is everywhere in nature but hard to pin down, because the rate and style of creep is sensitive to how the material was made, the tiny forces acting on it, and even background vibrations or noise. From a practical point of view, creep can be both good and bad. It can help smooth out weak spots in a material, a process that is used to make some metals stronger. But under the wrong conditions, it can also lead to sudden and catastrophic failures such as a landslide. Consider a pile of sand. It looks still, but the grains actually jiggle slightly with changes in air pressure and temperature. Over time, this jiggling can strengthen or weaken the sandpile, showing behavior that is surprisingly similar to other disordered solids like glass. The goal of this project is to understand what causes this creep in (disordered) materials like sand, toothpaste, or mud, using novel experimental methods and theoretical analysis. Experiments will track how particles inside these materials move under controlled forces that mimic nature, and will also measure how material strength changes as creep progresses. These data will be used to develop new computer models capable of predicting creep behavior, that will help to design new materials and prevent landslides. Finally, this project contributes to workforce development by training graduate and undergraduate students in soft condensed matter & materials research. The proposed work includes educational and outreach activities aimed at developing and engaging the local STEM community. Technical Abstract This proposal is about creep. The slow deformation of apparently solid materials that are both ubiquitous in nature and exquisitely sensitive to material preparation, local stresses, and noise. Creep may strengthen a given material as particle rearrangements ’iron out’ weak spots but may also weaken and lead to catastrophic failure when particle motion becomes more cooperative (e.g., avalanches) under some disturbances. Here, we aim to develop a universal understanding of creep in disordered materials by investigating the structure and dynamics of slow, sub-yield deformation in dense particulate systems -- and how these processes relate to bulk mechanical strength. We propose novel experimental investigations that traps creeping jammed suspensions at a 2D interface and in a 3D transparent bath, tunes particle interactions, and perturbs the samples with a spectrum of disturbances that relax or excite them. We will observe how materials deform at the particle level (i.e., sample microstructure) while simultaneously measuring the bulk rheology (e.g., creep compliance). We seek to relate sample microstructure to dynamics/rheology using the concept of excess entropy as an order parameter. This approach unites thermal and athermal systems and allows us to build a constitutive model that captures the pathologies of creep in disordered materials. Finally, the project aims to educate and train the next generation of physicists and engineers with expertise in soft-condensed matter, rheology, statistical physics, and mechanics. 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.