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Topic Name: Texas Chemical Engineers Identified New Way to Control the Motion of Fluid Particles Through Tiny Channels

Category: Chemical

Research persons: Dr. Thomas Truskett

Location: University of Texas, Austin, United States

Details

Chemical engineers at The University of Texas at Austin have discovered a new way to control the motion of fluid particles through tiny channels, potentially aiding the development of micro- and nano-scale technologies such as drug delivery devices, chemical and biological sensors, and components for miniaturized biological "lab-on-a-chip" applications.

The researchers learned that particle motion is strongly linked to how the particles arrange themselves in a channel.

“Particle arrangements are determined by the interactions of the particles with their boundaries. Thus, we were able to use these interactions as a means for controlling how readily the fluid will self-mix, diffuse, and flow,” said Dr. Thomas Truskett, associate professor of chemical engineering at the university.

The research by Ph.D. students Gaurav Goel, William Krekelberg and Truskett at the university along with Dr. Jeffrey Errington of the State University of New York at Buffalo, appears in the March 14 issue of the journal Physical Review Letters.

Civic planners and schoolteachers have long appreciated that the motion of cars on highways or children through hallways proceeds smoothly if lanes of traffic are formed. Truskett's research team found that a similar principle applies for the motion of fluid particles in narrow channels. Specifically, their computer simulations reveal that fluid particles move past one another more easily if they first form "layers" aligned with the boundaries of the channels.

The team has also introduced a way to systematically determine which types of channel boundaries will promote or frustrate the formation of the layers necessary for faster particle transport.

If layering leads to faster particle dynamics, it is natural to ask why bulk fluids adopt a more disordered structure with no layering, said Truskett.

“The reason: thermodynamics determines the structure of a fluid, not dynamics - and thermodynamics favors a disordered state for bulk fluids because it lowers the system's free energy,” he said.

The Truskett team determined that confining a fluid to small length scales allowed them to tune the thermodynamically-favored state to coincide with one that has layering and fast particle dynamics.

Truskett's latest research is funded by grants from the David and Lucile Packard Foundation, the Alfred P. Sloan Foundation, and the National Science Foundation. The Texas Advanced Computing Center and the University at Buffalo Center for Computational Research provided computational resources for this study.

Note for Fluid Mechanics
Fluid mechanics is the study of how fluids move and the forces on them. (Fluids include liquids and gases.) Fluid mechanics can be divided into fluid statics, the study of fluids at rest, and fluid dynamics, the study of fluids in motion. It is a branch of continuum mechanics, a subject which models matter without using the information that it is made out of atoms. The study of fluid mechanics goes back at least to the days of ancient Greece, when Archimedes made a beginning on fluid statics. However, fluid mechanics, especially fluid dynamics, is an active field of research with many unsolved or partly solved problems. Fluid mechanics can be mathematically complex. Sometimes it can best be solved by numerical methods, typically using computers. A modern discipline, called Computational Fluid Dynamics (CFD), is devoted to this approach to solving fluid mechanics problems. Also taking advantage of the highly visual nature of fluid flow is Particle Image Velocimetry, an experimental method for visualizing and analyzing fluid flow.

Like any mathematical model of the real world, fluid mechanics makes some basic assumptions about the materials being studied. These assumptions are turned into equations that must be satisfied if the assumptions are to hold true. For example, consider an incompressible fluid in three dimensions. The assumption that mass is conserved means that for any fixed closed surface (such as a sphere) the rate of mass passing from outside to inside the surface must be the same as rate of mass passing the other way. (Alternatively, the mass inside remains constant, as does the mass outside). This can be turned into an integral equation over the surface.
Fluid mechanics assumes that every fluid obeys the following:
Conservation of mass
Conservation of momentum
The continuum hypothesis, detailed below.
Further, it is often useful (and realistic) to assume a fluid is incompressible - that is, the density of the fluid does not change. Liquids can often be modelled as incompressible fluids, whereas gases cannot.
Similarly, it can sometimes be assumed that the viscosity of the fluid is zero (the fluid is inviscid). Gases can often be assumed to be inviscid. If a fluid is viscous, and its flow contained in some way (e.g. in a pipe), then the flow at the boundary must have zero velocity. For a viscous fluid, if the boundary is not porous, the shear forces between the fluid and the boundary results also in a zero velocity for the fluid at the boundary. This is called the no-slip condition.
Fluids are composed of molecules that collide with one another and solid objects. The continuum assumption, however, considers fluids to be continuous. That is, properties such as density, pressure, temperature, and velocity are taken to be well-defined at "infinitely" small points, defining a REV (Reference Element of Volume), at the geometric order of the distance between two adjacent molecules of fluid. Properties are assumed to vary continuously from one point to another, and are averaged values in the REV. The fact that the fluid is made up of discrete molecules is ignored.
The continuum hypothesis is basically an approximation, in the same way planets are approximated by point particles when dealing with celestial mechanics, and therefore results in approximate solutions. Consequently, assumption of the continuum hypothesis can lead to results which are not of desired accuracy. That said, under the right circumstances, the continuum hypothesis produces extremely accurate results.

The University of Texas at Austin's Cockrell School of Engineering ranks among the top six public engineering schools in the United States. With the nation's fourth highest number of faculty elected members of the National Academy of Engineering, the School's more than 7,000 students gain exposure to the nation's finest engineering practitioners. Appropriately, the School's logo, an embellished checkmark used by the first UT engineering dean to denote high quality student work, is the nation's oldest quality symbol.

Note for Thermodynamics
Thermodynamics is a branch of physics that studies the effects of changes in temperature, pressure, and volume on physical systems at the macroscopic scale by analyzing the collective motion of their particles using statistics. Roughly, heat means "energy in transit" and dynamics relates to "movement"; thus, in essence thermodynamics studies the movement of energy and how energy instills movement. Historically, thermodynamics developed out of need to increase the efficiency of early steam engines.

The starting point for most thermodynamic considerations are the laws of thermodynamics, which postulate that energy can be exchanged between physical systems as heat or work. They also postulate the existence of a quantity named entropy, which can be defined for any system. In thermodynamics, interactions between large ensembles of objects are studied and categorized. Central to this are the concepts of system and surroundings. A system is composed of particles, whose average motions define its properties, which in turn are related to one another through equations of state. Properties can be combined to express internal energy and thermodynamic potentials, which are useful for determining conditions for equilibrium and spontaneous processes.

With these tools, thermodynamics describes how systems respond to changes in their surroundings. This can be applied to a wide variety of topics in science and engineering, such as engines, phase transitions, chemical reactions, transport phenomena, and even black holes. The results of thermodynamics are essential for other fields of physics and for chemistry, chemical engineering, aerospace engineering, mechanical engineering, cell biology, biomedical engineering, and materials science to name a few.

In thermodynamics, there are four laws of very general validity, and as such they do not depend on the details of the interactions or the systems being studied. Hence, they can be applied to systems about which one knows nothing other than the balance of energy and matter transfer. Examples of this include Einstein's prediction of spontaneous emission around the turn of the 20th century and current research into the thermodynamics of black holes.

The four laws are:
Zeroth law of thermodynamics, stating that thermodynamic equilibrium is an equivalence relation.
If two thermodynamic systems are separately in thermal equilibrium with a third, they are also in thermal equilibrium with each other.
First law of thermodynamics, about the conservation of energy
The change in the internal energy of a closed thermodynamic system is equal to the sum of the amount of heat energy supplied to the system and the work done on the system.
Second law of thermodynamics, about entropy
The total entropy of any isolated thermodynamic system tends to increase over time, approaching a maximum value.
Third law of thermodynamics, about absolute zero temperature
As a system asymptotically approaches absolute zero of temperature all processes virtually cease and the entropy of the system asymptotically approaches a minimum value; also stated as: "the entropy of all systems and of all states of a system is zero at absolute zero" or equivalently "it is impossible to reach the absolute zero of temperature by any finite number of processes".
Onsager reciprocal relations (sometimes called the Fourth Law of Thermodynamics)
Express the equality of certain relations between flows and forces in thermodynamic systems out of equilibrium, but where a notion of local equilibrium exists.