Engineers discover predictor of mobility for fluids at nano-scale

Chemical engineers at The University of Texas at Austin have discovered a new way to predict the mobility of confined fluids at nanometer scales. At these scales, often just a few molecules across, fluids exhibit significantly different properties than at the macroscopic level.

The ability to predict these changes has applications in fields such as cell biology and geophysics, as well as important implications for the design of nanoscale devices.

The research by graduate student Jeetain Mittal and Dr. Thomas Truskett, assistant professor in the Department of Chemical Engineering at The University of Texas at Austin, along with Dr. Jeffrey Errington of the University at Buffalo, SUNY, appears in the May 5 issue of Physical Review Letters.

These results will help engineers understand how a variety of confined fluid systems function, from the performance of new materials for chemical separation and environmental remediation to transport processes in biological membranes. The discovery also provides a way to study the behavior of fluids in nanodevices, such as miniaturized “lab on a chip” tools for biomedical and analytical chemistry applications.

Confining fluids in very small, nanometer-scale channels can affect how the molecules pack together, how they withstand compression, and their ability to rapidly mix or flow. Changes to the first two properties are relatively well understood, but predicting the third, which is connected to the mobility of the molecules, has proven elusive.

“One of the most dramatic changes you see going from macroscopic scales to nanometer scales is that materials can actually change their state,” Truskett said. “A solid may become liquid upon confinement. If that solid material is a bonding agent and it turns into a runny fluid, it doesn’t do its job. Likewise, a liquid can become a solid when confined to small scales. If it is a lubricant, it fails. So in the engineering of nanoscale devices, these kinds of changes can have potentially catastrophic effects.”

In a bulk fluid, such as a glass of water, fluid molecules interact primarily with other fluid molecules. Relatively few are in contact with the surface of the container. At nanometer scales, however, a much higher proportion of molecules come in contact with the confining material. This surface interaction can significantly alter fluid properties, including molecular mobility.

The key to successfully predicting changes to mobility in a confined fluid, the researchers discovered, is the relationship between mobility and excess entropy.

“One way to think about how mobility relates to entropy is to think of entropy as measuring a sort of randomness at the molecular level,” Truskett said. “In a gas, where the molecules are randomly distributed, entropy is high and the gas mixes readily. In a solid, the molecules are aligned in a regular spatial pattern; there is little randomness and the solid barely mixes at all. Our discovery is that while both excess entropy and mobility of a fluid are affected by confinement, the relationship between the two quantities essentially remains the same down to very small scales.”

Because scientists already have reliable methods for predicting how confinement will affect excess entropy, they can now use this information together with the group’s findings to predict how confinement will affect fluid mobility.

The group performed computer simulations to study the behavior of fluids in highly restrictive channels with different shapes and boundary interactions. They were able to successfully model changes to fluid mobility and entropy in these conditions, a critical breakthrough that will allow engineers to learn how these changes occur while avoiding the difficult task of gathering experimental data on such small scales.

Source: University of Texas at Austin

Related stories:

Water flows like molasses on the nanoscale
A Georgia Tech research team has discovered that water exhibits very different properties when it is confined to channels less than two nanometers wide – behaving much like a viscous fluid with a viscosity approaching that of molasses. Determining the properties of water on the nanoscale may prove important for biological and pharmaceutical research as well as nanotechnology. The research appears in the March 15 issue of the journal Physical Review B.
Study shows how blood cells change shape
Millions of times during their four-month lifespan, human red blood cells must squeeze through tiny capillaries to deliver their payload of oxygen and pick up waste carbon dioxide-functions essential to life.
Macromolecules on surface control mobility in phospholipid bilayers
Phospholipid bilayers serve as the framework in biological membranes in which other components are embedded. Fundamental not only in biology, lipid bilayers are also essential in applications such as biosensors and nanoreactors.
New study identifies link between Alzheimer's disease biomarkers in healthy adults
A study published in the November issue of the Journal of Alzheimer's Disease provides an insight into normal, physiological levels and association between proteins involved in development of Alzheimer's disease. A group of scientists and physicians from the University of Washington and Puget Sound Veterans' Affairs Health Care System in Seattle, in collaboration with groups from the University of Pennsylvania and the University of California San Diego, performed a study in cognitively normal and generally healthy adults, from young to old (age range 21-88 years), of both genders, measuring levels of different brain-derived molecules associated with Alzheimer's disease.
Major North American breakthrough for dialysis patients
Suffering from end-stage renal disease (ESRD), a growing number of patients at the Centre hospitalier de l'Université de Montréal (CHUM), have become the beneficiaries of a North American breakthrough: high efficacy hemodiafiltration (HDF).
An emergency brake in the brain
Brain researchers at the University of Oslo in Norway have penetrated deeply into the innermost secrets of the brain to find out how brain cells can survive a stroke. Strokes are usually caused by occlusion of one of the blood vessels in the brain. When blood is prevented from supplying vital oxygen and energy to the brain cells, their electrochemical balance is upset, and they cause damage to themselves and to the surrounding brain cells before they collapse and die. Often this affects the memory centre, the hippocampus, where the cells are particularly vulnerable.
How do bacteria swim? Physicists explain
Imagine yourself swimming in a pool: It's the movement of your arms and legs, not the viscosity of the water, that mostly dictates the speed and direction that you swim.
Nontoxic nanoparticle can deliver and track drugs
A nontoxic nanoparticle developed by Penn State researchers is proving to be an all-around effective delivery system for both therapeutic drugs and the fluorescent dyes that can track their delivery.

News discussion:

Nanotechnology news