Have you ever opened a bottle of body lotion only to discover a watery liquid instead of a thick cream, or noticed that your hair gel behaves differently after some time than when it was new? The cause of these frustrating phenomena often lies in a hidden phenomenon known as the mechanical memory of materials. Namely, soft materials, such as gels, creams, and even some construction materials, possess a surprising ability to "remember" the processes they have gone through during production. This "memory," which manifests as residual internal stress, can persist for far longer than previously believed, which has significant consequences for the stability, durability, and predictability of countless products we use every day.

Revolutionary research conducted at the Massachusetts Institute of Technology (MIT) sheds a completely new light on this issue. Engineer Crystal Owens has developed an innovative, yet surprisingly simple method for measuring the degree of residual stress in soft materials. Her findings, published in the prestigious scientific journal Physical Review Letters, show that common products like hair gel or shaving cream retain their mechanical memory and internal stresses for weeks, and even months, which is in complete contrast to previous industry assumptions that measured this period in minutes.

The hidden life of soft glassy materials

Hand lotions, hair gels, shaving foams, but also mayonnaise, paints, and many pharmaceutical products belong to a fascinating category of materials known as soft glassy materials. These materials are unique hybrids that simultaneously exhibit properties of both solids and liquids. As Owens explains, "anything you can squeeze onto your palm and that forms a soft mound can be considered a soft glass." In materials science, they are considered a softer version of something that has an amorphous, unstructured molecular structure, similar to window glass. They can flow like a liquid, but at the same time, retain their shape like a solid.

It is this dual nature that makes them extremely useful, but also difficult to understand. After production, these materials exist in a delicate balance. The production process, which almost always involves some form of intense mixing, kneading, or shearing, introduces energy into the material. Although it seems that the material "settles down" and becomes stable after mixing, internal stresses remain trapped within its structure. These residual stresses represent a kind of imprint or "memory" of the forces to which it was exposed. Over time, the material can yield to these hidden forces and try to return to its previous, more unstable state, resulting in phase separation, a change in viscosity, or a complete loss of functionality.

A revolutionary method for measuring "memory"

Standard practice in industries such as cosmetics or food is to let a product sample rest for about one minute after mixing. Manufacturers have so far assumed that this time is sufficient for all residual stresses from the production process to dissipate and for the material to reach a stable, final state. However, Crystal Owens' research proves that this assumption was wrong.

Using a standard laboratory instrument known as a rheometer, Owens devised a new protocol for precisely measuring these long-lasting stresses. A rheometer consists of two plates between which a sample of the material is placed. By rotating and pressing the plates in a strictly controlled manner, the instrument can measure the internal resistance of the material, i.e., its stresses and strains. In her experiments, Owens placed samples of hair gel and shaving cream in the rheometer, mixed them to simulate an industrial process, and then left them to rest for significantly longer than the usual 60 seconds. During this long resting period, she continuously measured the tiny force the instrument had to apply to keep the material completely still. This force is a direct indicator of the magnitude of the internal stress that is trying to "push" the material back to its previous state.

The results were stunning. Not only did the materials retain a significant level of residual stress for days after mixing, but this stress was also directional. In other words, the material "remembered" the direction in which it had been mixed. If this stress were released, the gel would begin to deform in the direction opposite to the initial mixing. "The material can effectively remember in which direction it was mixed and how long ago," Owens points out. "It turns out that they retain this memory of their past for much, much longer than we thought."

From more stable creams to more durable roads

This discovery has enormous implications. It is one of the key reasons why different batches of the same product, produced by a seemingly "identical" process, can behave completely differently. Minimal variations in the speed, duration, or direction of mixing can result in different levels of residual stress, leading to inconsistent quality and durability of the product on the shelf. Understanding and measuring these hidden stresses during production could allow manufacturers to optimize their processes and design products that are significantly more stable and long-lasting.

In addition to the measurement protocol, Owens has also developed a mathematical model that can predict how a material will change over time based on the measured level of residual stress. Using this model, scientists could specifically design materials with "short-term memory" or very low residual stress, thus ensuring their long-term stability. This opens the door to innovations in numerous sectors.

One of the most promising areas of application is the construction industry, specifically the production of asphalt. Asphalt is a material that is first hot-mixed, then poured, and finally cools and hardens on the road. Owens suspects that residual stresses from the process of mixing aggregates and binders contribute significantly to the formation of cracks in the pavement over time. As the asphalt cools and is later subjected to daily and seasonal temperature changes, these internal stresses can lead to the appearance of microfractures that spread over time and become a serious problem. By reducing or controlling these initial stresses during the production phase, we could obtain significantly more resilient and durable roads.

"New types of asphalt are constantly being invented with the aim of being more environmentally friendly, and each of these new mixtures will have different levels of residual stress that will need to be controlled," says Owens. Potential applications extend even further, from optimizing printing pastes for 3D printing and developing more stable pharmaceutical ointments to improving the texture and shelf life of food products like yogurt or chocolate spreads. Understanding mechanical memory unlocks a new level of control over the world of materials that surrounds us.