Overview and Intellectual Merit

Goal: We were funded via the EAGER mechanism to explore inventing methods to fabricate glass grains with tunable sizes, shapes, and strengths. This exploration has converged on integrating new and established techniques from materials science, granular physics, signal processing, and photochemistry to convert a silicone polymer to glass. If successful, our invention will equip scientists with a vital tool for developing a unified theory of granular flow, one of the most challenging unsolved problems in physics.

Current techniques: Current state-of-the-art methods for developing a unified granular flow theory rely on experiments and simulations involving collections of grains with idealized shapes (e.g., spheres, disks, rods). The theories and frameworks developed from these approaches often fail when applied to heterogeneous flows induced by grain property variations. Few efforts have been made to control grain properties, and those undertakings have not produced crack- and void-free grains whose sizes, shapes, and strengths are tunable. We explored several methods for grain fabrication outlined in the initial proposal. Here, we describe the method we believe has the highest chance of success. This method includes molding liquid silicone into our desired shape and sizes, using heat to transform the polymer into a rubber-like solid, and then using light, heat, and oxygen to transform this rubber-like solid into glass. 

Designing and fabricating silicone grains
Work done: We have developed the experimental and coding pipelines to fabricate silicone grains of various shapes and sizes, ranging from 1–2 mm in diameter. Besides creating spherical grains, our work has included developing new methods or modifying existing ones to extract grain surfaces from (1) X-ray microtomography images of natural, non-spherical sand grains and (2) grains that we design using Fourier Descriptors—a method that models surfaces as a summation of sine waves with varying frequencies. We tessellate the grain surfaces and use them as depressions in eggshell-like molds that we 3D-print. We have used pipettes to fill the molds with degassed Polydimethylsiloxane (PDMS), a silicone polymer that is initially a viscous fluid and solidifies with heat. Unwanted solid materials, known as flashing, appear where the mold halves meet, which led us to explore several mold designs aimed at reducing flashing. The most successful designs include a depression surrounding the grains that collects excess material.

Future Possibilities: We have identified additional ways to improve the process described above. [1] We can enhance the resolution of the molds by printing them with a high resolution Two-Photon Polymerization printer. This endeavor might prove costly, but the molds will be re-usable. [2] We have also designed but not yet tested using a Computer Numerical Control (CNC) machine to automate the mold-filling process. The need for deflashing might be eliminated if we fill the clamped, eggshell-like molds through a small opening at the top of the mold.

Converting silicone grains to glass
Work done: Using existing photochemical theoretical and experimental research, we have arrived at a method to use deep ultraviolet (DUV) light, oxygen, and heat to catalyze reactions that convert PDMS to silica glass. The chemical elements required to produce silica (SiO₂) already exist within PDMS polymers, CH₃[Si(CH₃)₂O]ₙSi(CH₃)₃. The transformation process involves creating and maintaining an oxygen-rich environment so that the energy from DUV light continuously generates singlet atomic oxygen, O(¹D). These atoms destabilize the methyl groups (CH₃) in PDMS, replacing them with hydroxyl groups (OH). The heat generated by the light and reactions, reaching 220–300°C, dehydrates the hydroxyl groups, leaving behind fused silica with its characteristic Si–O–Si bonds.

We have purchased the equipment, built a custom experimental table, and secured the room to conduct this experiment successfully. The end of this grant coincides with the beginning of our initial tests of the procedure described above. These experiments should last six hours and produce grains with the mechanical and optical properties of glass. We predict that by optimizing the experimental parameters and incorporating post-UV irradiation heating, we will be able to fine-tune the grains’ strength, density, and size.

Broader Impacts

Importance: Granular materials are among the most ubiquitous materials found, used, and manipulated in natural and industrial settings, yet their behavior remains unpredictable. In nature, granular flows are the key drivers of geohazards, such as landslides, submarine slides, pyroclastic density currents, liquefaction, and earthquakes. In industrial settings, challenges arise in optimizing the storage, transport, and handling of grains, powders, and other aggregates. The complexity of granular flow processes stems from the fact that grain properties can combine to create near infinitely many fabrics and force networks that govern if, when, and how these materials flow. Our work addresses this challenge by developing a method to fabricate grains with tunable properties so that scientists can conduct controlled granular flow experiments to develop a unified theory of granular flow.

Student Training: A total of 7 trainees were trained in interdiciplinary science as a part of this grant.

Last Modified: 04/27/2025
Modified by: Vanshan Desmond Wright