Past soft materials retain "reminiscences" of their past history for a longer period than what was earlier believed.

In a groundbreaking study published in Physical Review Letters, Crystal Owens, a postdoctoral researcher at MIT's Computer Science and Artificial Intelligence Laboratory, has revealed that common products like hand lotion, hair gel, and shaving cream, as well as paving materials such as asphalt, hold onto residual stresses for extended periods.

The study, titled "Twist and Hold: Measuring the Relaxation of Mechanical Memory in Soft Glasses," suggests that understanding and measuring these hidden stresses during the manufacturing process could lead to the design of better products that last longer and perform more predictably.

Owens' research focuses on soft glassy materials, substances that exhibit properties of both solids and liquids. These materials, which can be poured like a liquid and hold their shape like a solid, are often used in our daily lives, from hand cream to hair gel.

In the manufacturing of such materials, Owens suspected that residual stress from the production process may persist long after the materials appear to settle. To test her hypothesis, she conducted experiments using a rheometer, an instrument that measures internal stresses and strains.

Owens found that the degree of stress that a material retained was a reflection of the direction in which it was initially mixed, and when it was mixed. If a gel is mixed in one direction, it holds onto the memory of that direction, causing internal stress that can cause the gel to shift in the opposite direction over time.

This discovery is significant because it means that manufacturers may not fully understand the long-term behaviour of the products they produce. For instance, if a hand cream is mixed in a way that causes it to retain significant residual stress, it may separate and become runny over time, affecting its performance and shelf life.

Owens also sees room for improvement in asphalt, a substance where she suspects residual stresses from the mixing process may contribute to cracks forming in pavement over time. Reducing these stresses at the start of the asphalt production process could lead to longer-lasting, more resilient roads.

To address this issue, Owens developed a model to estimate how a material will change over time, given the degree of residual stress that it holds. Using this model, scientists might design materials with "short-term memory," or very little residual stress, such that they remain stable over longer periods.

This research was supported, in part, by MIT's Postdoctoral Fellowship for Engineering Excellence and an MIT Mathworks Fellowship. Owens' work underscores the importance of understanding the hidden properties of the materials we use every day, from our personal care products to the infrastructure that supports our cities.