Soft materials hold onto ‘memories’ of their past for longer than previously thought

If your hand lotion is a little more cut than usual when leaving the bottle, it might have something to do with the “mechanical memory” of Goop.

Soft gels and lotions are made by mixing ingredients until they form a stable and uniform substance. But even after a frost, it can keep “memories” or a residual constraint, the mixture process. Over time, the material can yield to these integrated constraints and fall back into its old premedy state. Mechanical memory partly explains why hand lotion separates and becomes flowing over time.

Now, a MIT engineer has designed a simple way to measure the degree of residual stress in soft materials after being mixed, and found that current products such as hair gel and shaving cream have longer, by holding residual constraints for longer periods than manufacturers could have supposed.

In a study appearing in Physical examination lettersCrystal Owens, a postdoc in the IT and artificial MIT (CSAIL) computer laboratory, presents a new protocol to measure residual stress in soft and gel -like materials, using a standard benchtop rheometer.

By applying this protocol to flexible daily materials, Owens found that if a gel is made by mixing it in a direction, once it is installed in a stable and uniform state, it effectively retains the memory of the direction in which it is mixed. Even after several days, the frost will hold internal stress which, if it is released, will lead to a change of frost in the direction opposite to the way in which it was initially mixed, returning to its previous state.

“This is one of the reasons why different lots of cosmetics or food behave differently even if they have undergone an” identical “” manufacture, explains Owens. “Understanding and measuring these hidden constraints during treatment could help manufacturers design better products that last longer and operate more predictable.”

A soft glass

Hand lotion, hair gel and razing cream all come from the category of “soft glass materials” – materials that have solid and liquid properties.

“Everything you can pour into your hand and it forms a soft mound will be considered a soft glass,” explains Owens. “In materials science, it is considered a gentle version of something that has the same amorphous structure as glass.”

In other words, a gentle glassy material is a strange amalgam of a solid and a liquid. It can be poured like a liquid, and it can maintain its shape as a solid. Once manufactured, these materials exist in a delicate balance between the solid and the liquid. And Owens wondered: for how long?

“What happens to these materials after very long time? Do they finally relax or do they never relax?” Said Owens. “From the point of view of physics, it is a very interesting concept: what is the essential state of these materials?”

Twist

In the manufacture of gentle glass materials such as hair gel and shampoo, the ingredients are first mixed in a uniform product. The quality control engineers then let a sample stand for about a minute – a period of time which they suppose sufficient to allow the dissipation of the residual constraints of the mixture process. Meanwhile, the material must settle in a stable and stable state, ready to use.

But Owens suspected that the materials could hold a certain degree of stress of the production process long after they seemed to settle.

“Residual stress is a low level of stress that is trapped inside a material after arriving at the equilibrium state,” explains Owens. “This type of stress has not been measured in this type of material.”

To test her hypothesis, she carried out experiments with two common green materials: hair gel and shaving cream. It has made measurements of each material in a rheometer – an instrument made up of two rotary plates which can twist and support a material together to precision controlled pressures and forces which relate directly to the internal constraints and strains of the material.

In her experiences, she placed each material in the rheometer and turned the upper plate of the instrument to mix the material. Then she let the equipment settle, then settle a little more – a little more than a minute.

Meanwhile, she observed the amount of force that it took the rheometer to maintain the material in place. She considered that the greater the strength of the rheometer, the more it had to counter any constraint in the material which, otherwise, would bring it out of its current state.

During several experiments using this new protocol, Owens found that different types of gentle glass materials contained a significant amount of residual stress, long after most researchers assumed that stress had dissipated. In addition, she found that the degree of stress that a preserved material was a reflection of the direction in which it was initially mixed and when it was mixed.

“The material can actually” remember “which direction it was mixed and how long,” explains Owens. “And it turns out that they have this memory of their past, much longer than we think.”

In addition to the protocol she has developed to measure residual stress, Owens has developed a model to estimate how a material will change over time, given the residual degree of stress that he holds. By using this model, she says that scientists could design materials with a “short -term memory” or very little residual stress, so they remain stable over longer periods.

A material where it sees room for such an improvement is asphalt – a substance which is first mixed, then paid in land form on a surface where it cools and then settles over time. She suspects that the residual constraints of the mixture of asphalt can contribute to the formation of cracks in the road over time. The reduction of these constraints at the start of the process could lead to more sustainable and more resilient roads.

“People invent new types of asphalt all the time to be more environmentally friendly, and all these levels will have different levels of residual stress that will need certain control,” she said. “There is a lot of room to explore.”

This story is republished with the kind authorization of Mit News (Web.mit.edu/newsoffice/), a popular site that covers news of research, innovation and MIT teaching.