Researchers used strong X-ray beam technology to investigate what causes soft materials like toothpaste and hair gel to relax. Their findings can help in the development of new consumer items and nanotechnologies.
Close-up picture of a gel dripping from a cosmetic pipette. Gels are one type of soft material. These materials can easily deform in response to stress. Understanding the dynamics that affect how they relax is an active area of research. Image Credit: Shutterstock/Anastasiya Shatyrova.
Shampoo, shaving gel, and a cup of yogurt are all examples of soft materials, which change shape easily when stressed. Soft materials can alter shape under stress due to the small fluctuations of their dynamic particles.
This “relaxation” process happens randomly and on a very small scale for researchers to easily pinpoint. Investigators are understanding more about these materials, thanks to the Advanced Photon Source (APS), a US Department of Energy (DOE) Office of Science user facility housed at DOE’s Argonne National Laboratory.
Two distinct research groups successfully exploited a strong X-ray beam technique at the APS to unearth new knowledge about the dynamics of soft materials in a pair of recently published papers.
The understanding they gained could help in the design and advancement of a broad range of consumer products, including ice cream and gelatin desserts, personal care items like shampoo and moisturizers, batteries, foams, paints, and plastics used in manufacturing, and even nanotechnologies that make up coatings and drug delivery systems.
Understanding the dynamics of soft materials is important because we believe they have a direct and profound impact on properties we would want to control, such as viscosity and elasticity. Those properties control things like how soft a gel is or how fast a material flows.
Qingteng Zhang, Assistant Physicist, Argonne National Laboratory
Qingteng Zhang is a co-author of both papers.
How Researchers Leveraged the Power of X-Rays
The X-ray beam technique utilized in the research is known as X-ray photon correlation spectroscopy (XPCS). Through the use of such techniques, researchers may examine the structure and properties of a wide variety of materials at the atomic and molecular levels.
The objective of XPCS is to display microscopic dynamics in regions as small as a human hair’s diameter. They can record how dynamics change over time from as little as one-millionth of a second to several hours.
A substance is exposed to X-ray beams during the process. The characteristics of the X-Ray beams, like their direction of travel, vary when they hit the moving particles in the sample. The ability to measure how quickly particles in the material are moving over a range of lengths allows scientists to gain insight into the dynamics of the material’s structure.
Stress Relaxation in Hydrogels
Researchers from Argonne and Massachusetts Institute of Technology (MIT) published the XPCS research in the journal Proceedings of the National Academy of Sciences.
You can look at dynamics, or how things change in time, in two ways—on the small scale and the big scale. In our lab at MIT we use mechanical instruments called rheometers to look at changes on the larger scale, and then combined this with XPCS at the APS to understand the dynamics at the microscopic level.
Gareth McKinley, Study Co-Author and Professor, Massachusetts Institute of Technology
Researchers examined the dynamics of a hydrogel with and without external mechanical stress to obtain a thorough picture of its dynamics. This made linkages between the material’s small- and large-scale changes more apparent.
XPCS helped us understand the microscopic rearrangements that occur inside soft gel materials, especially in the presence of mechanical stress. This has implications for designing soft materials ranging from hydrogels used in drug delivery and cell culture, to emulsions and pastes used in consumer products.
Jake Song, Study Lead Author and Graduate Student, Massachusetts Institute of Technology
Soft Material Relaxation at the Interfaces
Researchers from Argonne, Berkeley Lab (a DOE facility), and University of Massachusetts (UMASS) Amherst used XPCS to analyze soft materials in different research that was published in the journal ACS Nano. However, in this instance, scientists were looking at an oil and water mixture.
Scientists placed very thin particles called nanoparticles between the surfaces of the two liquids. Particles were likely to jam or become more closely packed, and then bond to create solid-like structures in this region. The researcher utilized XPCS to monitor dynamics when jamming was active.
“Ultimately what we got from XPCS was a better understanding of how jamming is moderated by the dynamics of the system, which are insights we could use in the future to make liquid structures that behave in a particular way,” states Tom Russell, Study co-author and Professor at the University of Massachusetts Amherst. Tom Russell is also a visiting scientist at Berkeley Lab.
Future of X-Ray Tools at the APS
As the APS is currently being upgraded, scientists may be able to get even more out of techniques like XPCS in the future.
The improved APS will greatly enhance X-ray beam coherence, or how well the rays’ wavefronts are synchronized, and this specific approach will increase by up to a million times as a result.
Zhang concludes, “These upgrades will greatly expand the types of materials we can measure with this technique in the future. It will be exciting to see the new science the APS can enable in years to come.”
The study published in ACS Nano was supported by the Office of Basic Energy Sciences within DOE’s Office of Science, the National Science Foundation, and the DOE Office of Science Graduate Research Program.
The research published online in the Proceedings of the National Academy of Sciences was funded by the National Science Foundation, the US Army Research Office, and DOE.
Kim, P. Y., et al. (2022) Relaxation and Aging of Nanosphere Assemblies at a Water–Oil Interface. ACS Nano. doi.org/10.1021/acsnano.2c00020.
Song, J., et al. (2022) Microscopic dynamics underlying the stress relaxation of arrested soft materials. Proceedings of the National Academy of Sciences. doi.org/10.1073/pnas.2201566119.