Recently researchers at MIT have developed a new method for determining the structure and behavior of a class of widely used soft materials known as weak colloidal gels, which are found in everything from cosmetics to building materials. The study characterizes the gels over their entire evolution, as they change from mineral solutions to elastic gels and then glassy solids.
The work uncovers the micro structural mechanisms underlying how the gels change naturally over time, and how their elastic properties also change, both over time and depending on the rate at which they are experimentally deformed. This characterization should allow further study, prediction, and perhaps manipulation of the gels’ behavior, opening doors to advances in such areas as drug delivery and food production, in which these gels are common ingredients, as well as in applications ranging from water purification to nuclear waste disposal, which use these colloidal gels in a crystallized, porous form known as zeolites.
We believe this new overall picture and understanding of the gelation and subsequent aging process is of great importance for material scientists who work on soft matter, says Gareth McKinley, the School of Engineering Professor of Teaching Innovation and professor of mechanical engineering at MIT.
Our results enable researchers to determine why weak colloidal gels show aspects of both glassy and gel-like behavior, and to possibly engineer the gels to have particular desired features in their mechanical response, says Bavand Keshavarz, a postdoc in MIT’s Department of Mechanical Engineering and first author of the new study, which appears inPNAS. The research was performed as part of an international collaboration involving MIT, Argonne National Laboratory, the French National Center for Scientific Research, and the French Alternative Energies and Atomic Energy Commission. Using aluminosilicate gels, widely utilized for making zeolites, the researchers overcame many of the challenges associated with characterizing these very soft materials, which continuously change over time, as well as exhibiting different properties depending on the rate at which that are deformed. Keshavarz likens their behavior to that of Silly Putty, which stretches and flows if you pull it slowly, but breaks off sharply if you give it a fast tug.
The resultant snapshots provided a comprehensive profile of the mechanical properties of the gels, allowing the researchers to conclude that weak colloidal gels, also known colloquially as pasty materials, have a dual nature, exhibiting features of both glasses and gels. Prior to this study, researchers’ limited observational perspectives led them to conclude that such materials were either gels or glasses, not having observed both features in a single experiment.
To observe the evolving structure of aluminosilicate gels, in addition to examining their mechanical properties throughout the gelation and aging process, the researchers applied X-ray scattering. This allowed them to resolve the structure of the gel starting from when its chemical components were smaller than the wavelength of light and therefore invisible without the penetration of X-rays. The process allowed the researchers to observe the physical structure of the gels at length scales ranging over four orders of magnitude, zooming in from a scale of 1 micron down to that of 0.1 nanometer. Observing the gels at such wide-ranging spatial scales, the researchers discovered that the fractal-like network of connected particles that develops as the particles cluster into a gel remains fixed beyond the gel point. The network grows and adds clusters, changing in scale, but the main structure or “backbone” and geometry remain the same.
Examining the materials over such widely-ranging spatial scales and combining this information with the concurrent information about the materials’ mechanical behavior, the researchers also concluded that larger clusters within the network relaxed more slowly in a gel-like manner after being deformed while the smaller clusters relaxed more quickly like a rigid glassy material. McKinley makes the analogy to the marked differences we experience between the time it takes for a memory foam mattress to recover from being compressed versus the time a very hard conventional mattress takes. Observing this relationship between the size of clusters within the material and the rate of relaxation sheds further light on the origins of these soft materials distinctive properties. Our work opens up a novel perspective,” says Keshavarz, “and paves the path for researchers to develop a more comprehensive view about the nature of these pasty materials. Colloidal gels are ubiquitous materials,” says Emanuela Del Gado, associate professor in Georgetown University’s Department of Physics, who was not involved in this research but has collaborated with the MIT team in the past. Their physics is important in so many industries and technologies (from food to paint, to cement, personal care products and biomedical applications). This paper is the first attempt to identify the microscopic traits that unify the mechanics of a potentially wide class of systems, by connecting [the gels’] microstructure to their rheological behavior.