Vibration Isolation Supports the Search for Memory in Soft, Amorphous Solids

The Soft Mechanics Laboratory at California Polytechnic State University (Cal Poly is researching soft, amorphous solids like foams, concentrated emulsions and bulk metallic glasses to determine whether these materials remember deformations that were applied repeatedly in the past, and how this information could be encoded in their microscopic structure and behaviors.

Nathan Keim, Assistant Professor of Physics at Cal Polys Soft Mechanics Laboratory in San Luis Obispo, has for well over a decade been conducting research on memory, self-organization and other non-equilibrium behaviors of soft materials under deformation.

This includes experimental studies of the mechanisms of plasticity in disordered solids, and liquid interfaces. With his students, the lab conducts experiments on interfacial materials, made up of microscopic particles adsorbed at an oil-water interface, using these two-dimensional systems as models of soft, amorphous solids like foams, concentrated emulsions and bulk metallic glasses.

Amorphous solids like foam, sand, and ice cream feature many particles crowded together. Each particle is fully constrained by its neighbors, but the material is disordered so these constraints vary greatly, said Professor Keim. When the material is put under sufficient stress, we know that local groups of particles rearrange by squeezing past one another, but the way these microscopic processes organize and give rise to macroscopic behaviors of the material is still not well understood.

We focus on deformations that are large enough to cause these rearrangements and change the material, but too small to disrupt the material completely and cause it to start flowing. explained Professor Keim. This creates the possibility that the material can retain a detailed memory of how it was deformed in the past. Exploring how these memories are stored and how they can be retrieved is an opportunity to learn more about the mechanisms at play when these materials deform.

Interfacial Shear Rheometer and Managing Vibrations
To facilitate this research, Professor Keim and his team have constructed an instrument, a kind of interfacial shear rheometer, which allows a two-dimensional amorphous solid to be deformed while measuring its mechanical properties.

The apparatus uses a pair of magnetic coils to move a magnetized needle embedded in the material, deforming the material with pN (piconewton) forces. By simultaneously recording the motion of tens of thousands of particles in the material, they can connect what is happening on the microscopic and macroscopic scales a rare and highly desirable ability in the study of materials.

Since our research basically takes place on the surface of a dish of water, the experiment is especially sensitive to low-frequency vibrations that excite surface waves and agitate the material under study, added Professor Keim. The building where the physics department, and the Soft Mechanics Lab is housed, has severe vibrations at 5 Hz due to the many fume hoods in the building, as well as the HVAC system.

When these vibrations are too strong, they do not just introduce noise, but actually trigger rearrangements of the particles within the sample material essentially altering the sample during the measurement. Even when they do trigger rearrangements, low-frequency disturbances also add noise to the mechanical measurements and limit the ability to deform the material precisely and repeatedly

For vibration isolation, the experiments were being conducted on a tabletop platform resting on passive balloon isolators. But this was not sufficient to dampen vibrations satisfactorily. So the lab began performing experiments at night when the chemistry fume hoods were not in operation, which helped mitigate the problem, but still required considerable computer processing afterwards to stabilize imaging.

These vibration limitations had grown more important as we began to examine our materials response when the exact same deformation is applied hundreds of times, in order to study the possibility of learning and memory effects, continued Professor Keim.

Having looked at both optical air tables and active vibration cancellation systems as potential solutions for the vibration problem, and determined that neither was satisfactory for cancelling vibrations at very low frequencies, the lab selected Negative-Stiffness vibration isolation.

2D solid material sample: A layer of polystyrene particles at the interface between oil and water. The black bars at the top and bottom are the boundaries that are moved to shear the material.

The Natural History Museums of Los Angeles County (NHMLAC), which include the Natural History Museum in Los Angeles' Exposition Park, the La Brea Tar Pits, and the William S. Harding Museum, is the largest natural and historical museum in the western United States, holding one of the world’s most extensive and valuable collections of natural and cultural history—more than 35 million specimens and artifacts covering 4.5 billion years of history.

The NHMLAC collections are strong in many fields, but the mineralogy and Pleistocene paleontology collections are among the most impressive, the latter thanks to the wealth of specimens collected from the La Brea Tar Pits located in the heart of Los Angeles. The worlds most powerful gateway to the Ice Age, the asphalt seeps at the La Brea Tar Pits represent the only active urban fossil dig site in the world. The site contributes an ongoing wealth of extraordinary specimens, like saber-toothed cats, mammoths, dire wolves, and mastodons, as well as the tiny microfossils of insects, plants, mammals, and reptiles from the last 50,000 years.