Be it laundry detergent, paint, ketchup or salad dressing, even before it leaves the production line, gravity begins tugging at it, trying to separate the mixture into different parts. As products separate, they can become watery or gooey. To combat product collapse and increase shelf life, manufactures use stabilizers.
The Advanced Colloids Experiment (ACE-M-1) is designed to help researchers understand how to optimize stabilizers to extend product shelf life, while also cutting development, production and transportation costs. ACE-M-1 launched to the International Space Station on the second commercial resupply flight of the SpaceX Dragon on March 1.
"When these products sit on a shelf for a certain amount of time, they start coarsening and separating," said Bill Meyer, ACE-M-1 project scientist at NASA's Glenn Research Center in Cleveland. "You see a top half and a bottom half that are different. Stabilizers keep the product doing what you want it to do. While we have a general understanding about what is happening at the particle level with these stabilizers, there's a lot more that we need to know."
On Earth, several things happen simultaneously. For example, depending on the mixture, particles move around and settle at the same time. As they coarsen or evolve and change, heavier particles can settle to the bottom within only a few minutes -- a process called sedimentation. Such complexity makes it hard to observe the underlying physics. Microgravity allows scientist to create models and develop more universal theories by essentially slowing down the separation process.
"We can do things in space we can't do on Earth," Meyer said. "Gravity masks the effect we're looking for. In space you won't see a top and bottom half since that's caused by gravity and density differences. What you'll see on the International Space Station is little blobs form. And those little blobs will grow over time. In microgravity we can measure this coarsening, or evolution, since gravity's influence is about a million times weaker."
Matthew Lynch, ACE-M-1, principal investigator and principal scientist at Procter & Gamble (P&G) in Cincinnati, describes it as a house-of-cards. Solid particles organize into a network that supports weight and counteracts the tendency separate.
"If you remove too many cards, the structure collapses," Lynch said. "Coarsening is about the rate at which the cards move around. There are some rudimentary theories that allow us to treat such time scales where all the cards are the same. However, we can't anticipate the fall of the house-of-cards when the cards are very different. Further, we do not have a framework to understand the movement altogether. For example, do we pull cards out of the house altogether and place them on top, or do we shuffle them around other cards in the same locale."
P&G, an international producer of scores of consumer products, is using the ACE-M-1 investigation to study product stability on the space station. P&G products like liquid detergent, shampoos, cleaners and medicine are colloidal systems. Anything that has particles of one micron or less in suspension is known as a colloid system. To give an idea of the size of a colloid, a very fine human hair is about 100 microns in diameter.
In colloidal systems, heavier particles settle to the bottom while lighter ones float to the top. Colloidal gels make up the microstructure of many consumer products such as detergents and shampoos. These gels are often polydisperse, which is where little particles are not just one size but a range of sizes. To control these systems, scientists need an understanding of the coarsening of the microstructure.
"We're doing research on the International Space Station because it teaches all about our stabilizing systems and our products," said Lynch. "Stabilizers keep everything together to make sure that when somebody buys a product and uses it, it keeps all the material basically uniform throughout the product."
With the effects of gravity removed, ACE-M-1 will allow scientists to use the Light Microscopy Module (LMM) to observe what happens at the particle level. A remotely controllable, automated microscope, the LMM gives scientists the ability to study specimens in microgravity in real time while the interesting science is happening. The LMM, which operates in the Fluids & Combustion Facility (FCF), resides in the space station's Destiny Laboratory and is managed by NASA's Glenn Research Center.
The FCF is a refrigerator-size rack, about 6 feet tall. The sample module inside the rack is approximately the size of a deck of cards. The ACE-M-1 sample itself is only a couple drops, measuring a couple microliters in volume. The investigation will run several experiments throughout its months aboard the space station.
"We do several things in space," said Ron Sicker, ACE-M-1 project manager, NASA's Glenn Research Center. "Some of the things we do are theoretical. The payoff may be big, but it may be many years in the future before it is realized. The ACE-M-1 investigation we're doing with Procter and Gamble holds open both the possibility for a long-term, theoretical win, as well as a win within just a few years as it finds its way into everyday products."
Better stabilizers could stack the deck in our favor. It could mean many things for manufacturers and consumers, including better quality, reduced costs and greener, more concentrated products that use less plastic in their packaging. It could mean products that resist collapse to remain consistent throughout their life, where the first ounce coming out of the bottle is the same as the last.
NASA's Glenn Research Center