Making complex fluids look simple

ScienceDaily (June 1, 2011) — An international research team has successfully developed a widely applicable method for discovering the physical foundations of complex fluids for the first time. Researchers at the University of Vienna and University of Rome have developed a microscopic theory that describes the interactions between the various components of a complex polymer mixture. This approach has now been experimentally proven by physicists from Jülich, who conducted neutron scattering experiments in Grenoble.

The results have been published in the June issue of the journal Physical Review Letters.

Some important materials from technology and nature are complex fluids: polymer melts for plastics production, mixtures of water, oil and amphiphiles, which can be found in both living cells and in your washing machine, or colloidal suspensions such as blood or dispersion paints. They are quite different from simple fluids consisting of small molecules, such as water, because they are made of mixtures of particles between a nanometre and a micrometre in size, and have a large number of so-called degrees of freedom. The latter include vibrations, movements of the functional groups of molecules or joint movements of several molecules. They can appear on widely varied length, time, and energy scales. This makes experimental and theoretical studies difficult and, so far, has impeded understanding of the properties of these systems and the targeted development of new materials with improved properties.

A method developed and tested by physicists at Forschungszentrum Jülich, the Institut Laue-Langevin in Grenoble, and the Universities of Vienna and Rome now permits realistic modelling of complex fluids for the first time. "Our microscopic theory describes the interactions between the various components of a complex mixture and in turn, enables us to draw realistic conclusions about their macroscopic properties, such as their structure or their flow properties," said Prof. Christos Likos of the University of Vienna, an expert on theory and simulation.

The team from Vienna and Rome developed the theory model. Since the researchers were unable to include all the details of the real system -- a mixture of larger star-shaped polymers and smaller polymer chains -- they systematically eliminated the rapidly moving degrees of freedom and focused on the relevant slow degrees of freedom, a time-consuming and challenging task. "To do this, we use a relatively new method called coarse graining and replace each complex macromolecule with a sphere of the appropriate size. The challenge involves integrating the degrees of freedom that have been eliminated in the simplified systems as averages so that the characteristics of the substances are retained," Likos explained.

The team from Jülich used elaborate small angle neutron scattering experiments with the instrument D11 at the Institut Laue-Langevin in Grenoble to prove that the interactions between the spheres of the coarse-grained model realistically simulate the conditions in the real system. "We were faced with the proverbial challenge of visualizing the needle in a haystack," explained Dr. Jörg Stellbrink, a physicist and neutron scattering expert at the Jülich Centre for Neutron Science (JCNS). For neutrons, the individual polymers of the mixture cannot be readily distinguished. For this reason, the physicists "coloured" the components they were interested in, so that they stood out of the crowd. This is one of the Jülich team's specialities. In this way, they were able to selectively examine the structures and interactions on a microscopic length scale.

The physicists are especially proud of the excellent agreement between theoretical predictions and experimental results. The method will now open up a spectrum of possibilities for studying the physical properties of a whole range of different complex mixtures.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Helmholtz Association of German Research Centres, via EurekAlert!, a service of AAAS.

Journal Reference:

B. Lonetti, M. Camargo, J. Stellbrink, C. Likos, E. Zaccarelli, L. Willner, P. Lindner, D. Richter. Ultrasoft Colloid-Polymer Mixtures: Structure and Phase Diagram. Physical Review Letters, 2011; 106 (22) DOI: 10.1103/PhysRevLett.106.228301

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Do microbes swim faster or slower in elastic fluids? Research answers long-standing question

ScienceDaily (May 18, 2011) — A biomechanical experiment conducted at the University of Pennsylvania School of Engineering and Applied Science has answered a long-standing theoretical question: Will microorganisms swim faster or slower in elastic fluids? For a prevalent type of swimming, undulation, the answer is "slower."

Paulo Arratia, assistant professor of mechanical engineering and applied mechanics, along with student Xiaoning Shen, conducted the experiment. Their findings were published in the journal Physical Review Letters.

Many animals, microorganisms and cells move by undulation, and they often do so through elastic fluids. From worms aerating wet soil to sperm racing toward an egg, swimming dynamics in elastic fluids is relevant to a number of facets of everyday life; however, decades of research in this area have been almost entirely theoretical or done with computer models. Only a few investigations involved live organisms.

"There have been qualitative observations of sperm cells, for example, where you put sperm in water and watch their tails, then put them in an elastic fluid and see how they swim differently," Arratia said. "But this difference has never been characterized, never put into numbers to quantify exactly how much elasticity affects the way they swim, is it faster or slower and why."

The main obstacle for quantitatively testing these theories with live organisms is developing an elastic fluid in which they can survive, behave normally and in which they can be effectively observed under a microscope.

Arratia and Shen experimented on the nematode C. elegans, building a swimming course for the millimeter-long worms. The researchers filmed them through a microscope while the creatures swam the course in many different liquids with different elasticity but the same viscosity.

Though the two liquid traits, elasticity and viscosity, sound like they are two sides of the same coin, they are actually independent of each other. Viscosity is a liquid's resistance to flowing; elasticity describes its tendency to resume its original shape after it has been deformed. All fluids have some level of viscosity, but certain liquids like saliva or mucus, under certain conditions, can act like a rubber band.

Increased viscosity would slow a swimming organism, but how one would fare with increased elasticity was an open question.

"The theorists had a lot of different predictions," Arratia said. "Some people said elasticity would make things go faster. Others said it would make things go slower. It was all over the map.

"We were the first ones to show that, with this animal, elasticity actually brings the speed and swimming efficiency down."

The reason the nematodes swam slower has to do with how viscosity and elasticity can influence each other.

"In order to make our fluids elastic, we put polymers in them," Arratia said. "DNA, for example, is a polymer. What we use is very similar to DNA, in that if you leave it alone it is coiled. But if you apply a force to it, the DNA or our polymer, will start to unravel.

"With each swimming stroke, the nematode stretches the polymer. And every time the polymers are stretched, the viscosity goes up. And as the viscosity goes up, it's more resistance to move through."

Beyond giving theorists and models a real-world benchmark to work from, Arratia and Shen's experiment opens the door for more live-organism experiments. There are still many un-answered questions relating to swimming dynamics and elasticity.

"We can increase the elasticity and see if there is a mode in which speed goes up again. Once the fluid is strongly elastic, or closer to a solid, we want to see what happens," Arratia said. "Is there a point where it switches from swimming to crawling?"

Arratia and Shen's research was supported by the National Science Foundation.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Pennsylvania.

Journal Reference:

X. Shen, P. Arratia. Undulatory Swimming in Viscoelastic Fluids. Physical Review Letters, 2011; 106 (20) DOI: 10.1103/PhysRevLett.106.208101

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Do microbes swim faster or slower in elastic fluids? Research answers long-standing question

ScienceDaily (May 18, 2011) — A biomechanical experiment conducted at the University of Pennsylvania School of Engineering and Applied Science has answered a long-standing theoretical question: Will microorganisms swim faster or slower in elastic fluids? For a prevalent type of swimming, undulation, the answer is "slower."

Paulo Arratia, assistant professor of mechanical engineering and applied mechanics, along with student Xiaoning Shen, conducted the experiment. Their findings were published in the journal Physical Review Letters.

Many animals, microorganisms and cells move by undulation, and they often do so through elastic fluids. From worms aerating wet soil to sperm racing toward an egg, swimming dynamics in elastic fluids is relevant to a number of facets of everyday life; however, decades of research in this area have been almost entirely theoretical or done with computer models. Only a few investigations involved live organisms.

"There have been qualitative observations of sperm cells, for example, where you put sperm in water and watch their tails, then put them in an elastic fluid and see how they swim differently," Arratia said. "But this difference has never been characterized, never put into numbers to quantify exactly how much elasticity affects the way they swim, is it faster or slower and why."

The main obstacle for quantitatively testing these theories with live organisms is developing an elastic fluid in which they can survive, behave normally and in which they can be effectively observed under a microscope.

Arratia and Shen experimented on the nematode C. elegans, building a swimming course for the millimeter-long worms. The researchers filmed them through a microscope while the creatures swam the course in many different liquids with different elasticity but the same viscosity.

Though the two liquid traits, elasticity and viscosity, sound like they are two sides of the same coin, they are actually independent of each other. Viscosity is a liquid's resistance to flowing; elasticity describes its tendency to resume its original shape after it has been deformed. All fluids have some level of viscosity, but certain liquids like saliva or mucus, under certain conditions, can act like a rubber band.

Increased viscosity would slow a swimming organism, but how one would fare with increased elasticity was an open question.

"The theorists had a lot of different predictions," Arratia said. "Some people said elasticity would make things go faster. Others said it would make things go slower. It was all over the map.

"We were the first ones to show that, with this animal, elasticity actually brings the speed and swimming efficiency down."

The reason the nematodes swam slower has to do with how viscosity and elasticity can influence each other.

"In order to make our fluids elastic, we put polymers in them," Arratia said. "DNA, for example, is a polymer. What we use is very similar to DNA, in that if you leave it alone it is coiled. But if you apply a force to it, the DNA or our polymer, will start to unravel.

"With each swimming stroke, the nematode stretches the polymer. And every time the polymers are stretched, the viscosity goes up. And as the viscosity goes up, it's more resistance to move through."

Beyond giving theorists and models a real-world benchmark to work from, Arratia and Shen's experiment opens the door for more live-organism experiments. There are still many un-answered questions relating to swimming dynamics and elasticity.

"We can increase the elasticity and see if there is a mode in which speed goes up again. Once the fluid is strongly elastic, or closer to a solid, we want to see what happens," Arratia said. "Is there a point where it switches from swimming to crawling?"

Arratia and Shen's research was supported by the National Science Foundation.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Pennsylvania.

Journal Reference:

X. Shen, P. Arratia. Undulatory Swimming in Viscoelastic Fluids. Physical Review Letters, 2011; 106 (20) DOI: 10.1103/PhysRevLett.106.208101

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Mixing fluids efficiently in confined spaces: Let the fingers do the working

ScienceDaily (May 13, 2011) — Getting two fluids to mix in small or confined spaces is a big problem in many industries where, for instance, the introduction of one fluid can help extract another -- like water pumped underground can release oil trapped in porous rock -- or where the mixing of liquids is the essential point of the process. A key example of the latter is microfluidics technology, which allows for the controlled manipulation of fluids in miniscule channels often only a few hundred nanometers wide.

Microfluidic devices were first introduced in the 1980s and for many years were best known for their use in ink-jet printers, but have since been introduced in other fields, including the chemical analysis of blood or other sera in lab-on-a-chip technologies. These devices -- usually not much larger than a stick of chewing gum -- sometimes rely on nano-sized moving components, the geometry of the grooved channels or pulsed injections to induce a mixing of the fluids. But researchers in MIT's Department of Civil and Environmental Engineering suggest that a simpler method might be equally, if not more, effective.

"Getting two fluids to mix in a very tight space is difficult because there's not much room for a disorderly flow," said Professor Ruben Juanes, the ARCO Associate Professor in Energy Studies and principal investigator on the research. "But with two fluids of highly contrasting viscosity, the thinner fluid naturally creates disorder, which proves to be a marvelously efficient means of mixing."

In an analysis published online May 12 in Physical Review Letters (PRL), the researchers show that the injection of a thin or low-viscosity fluid into a much more viscous fluid (think of water spurting into molasses) will cause the two fluids to mix very quickly via a physical process known as viscous fingering. The thinner liquid, say the researchers, will form fingers as it enters the thicker liquid, and those fingers will form other fingers, and so on until the two liquids have mixed uniformly.

They also found that for maximum mixing to occur quickly, the ideal ratio of the viscosity of any two fluids depends on the speed at which the thinner liquid is injected into the thicker one.

The research team of Juanes, postdoctoral associate Luis Cueto-Felgueroso and graduate students Birendra Jha and Michael Szulczewski, made a series of controlled experiments using mixtures of water and glycerol, a colorless liquid generally about a thousand times more viscous than water. By alternating the viscosity of the liquids and the velocity of the injection flows, Jha was able to create a mathematical model of the process and use that to determine the best viscosity ratio for a particular velocity. He is lead author on the PRL paper.

"It's been known for a very long time that a low viscosity fluid will finger through the high viscosity fluid," said Juanes. "What was not known is how this affects the mixing rate of the two fluids. For instance, in the petroleum industry, people have developed increasingly refined models of how quickly the low viscosity fluid will reach the production well, but know little about how it will mix once it makes contact with the oil."

Similarly, Juanes said, in microfluidics technology, the use of fluids of different viscosities has not been seriously proposed as a mixing mechanism, but the new study indicates it could work very efficiently in the miniscule channels.

"We can now say that on average, the viscosity of the fluid injected should be about 10 times lower than that of the fluid into which it is injected," said Juanes. "If the contrast is greater than 10, then the injection should be done more slowly to achieve the fastest maximum mixing. Otherwise, the low viscosity fluid will create a single channel through the thicker fluid, which is not ideal."

Cueto-Felgueroso said a similar process is at work in the engraved channels of a microfluidic device and in subsurface rock containing oil. "Mixing fluids at small scales or velocities is difficult because you can't rely on turbulence: it would be hard to stir milk into your coffee if you were using a microscopic cup," Cueto-Felgueroso said. "With viscous fingering, you let the fluids do the job of stirring."

This work was funded by the Italian energy company, Eni, and the ARCO Chair in Energy Studies.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Massachusetts Institute of Technology, Department of Civil and Environmental Engineering. The original article was written by Denise Brehm.

Journal Reference:

Birendra Jha, Luis Cueto-Felgueroso, Ruben Juanes. Fluid Mixing from Viscous Fingering. Physical Review Letters, 2011; 106 (19) DOI: 10.1103/PhysRevLett.106.194502

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Mixing fluids efficiently in confined spaces: Let the fingers do the working

ScienceDaily (May 13, 2011) — Getting two fluids to mix in small or confined spaces is a big problem in many industries where, for instance, the introduction of one fluid can help extract another -- like water pumped underground can release oil trapped in porous rock -- or where the mixing of liquids is the essential point of the process. A key example of the latter is microfluidics technology, which allows for the controlled manipulation of fluids in miniscule channels often only a few hundred nanometers wide.

Microfluidic devices were first introduced in the 1980s and for many years were best known for their use in ink-jet printers, but have since been introduced in other fields, including the chemical analysis of blood or other sera in lab-on-a-chip technologies. These devices -- usually not much larger than a stick of chewing gum -- sometimes rely on nano-sized moving components, the geometry of the grooved channels or pulsed injections to induce a mixing of the fluids. But researchers in MIT's Department of Civil and Environmental Engineering suggest that a simpler method might be equally, if not more, effective.

"Getting two fluids to mix in a very tight space is difficult because there's not much room for a disorderly flow," said Professor Ruben Juanes, the ARCO Associate Professor in Energy Studies and principal investigator on the research. "But with two fluids of highly contrasting viscosity, the thinner fluid naturally creates disorder, which proves to be a marvelously efficient means of mixing."

In an analysis published online May 12 in Physical Review Letters (PRL), the researchers show that the injection of a thin or low-viscosity fluid into a much more viscous fluid (think of water spurting into molasses) will cause the two fluids to mix very quickly via a physical process known as viscous fingering. The thinner liquid, say the researchers, will form fingers as it enters the thicker liquid, and those fingers will form other fingers, and so on until the two liquids have mixed uniformly.

They also found that for maximum mixing to occur quickly, the ideal ratio of the viscosity of any two fluids depends on the speed at which the thinner liquid is injected into the thicker one.

The research team of Juanes, postdoctoral associate Luis Cueto-Felgueroso and graduate students Birendra Jha and Michael Szulczewski, made a series of controlled experiments using mixtures of water and glycerol, a colorless liquid generally about a thousand times more viscous than water. By alternating the viscosity of the liquids and the velocity of the injection flows, Jha was able to create a mathematical model of the process and use that to determine the best viscosity ratio for a particular velocity. He is lead author on the PRL paper.

"It's been known for a very long time that a low viscosity fluid will finger through the high viscosity fluid," said Juanes. "What was not known is how this affects the mixing rate of the two fluids. For instance, in the petroleum industry, people have developed increasingly refined models of how quickly the low viscosity fluid will reach the production well, but know little about how it will mix once it makes contact with the oil."

Similarly, Juanes said, in microfluidics technology, the use of fluids of different viscosities has not been seriously proposed as a mixing mechanism, but the new study indicates it could work very efficiently in the miniscule channels.

"We can now say that on average, the viscosity of the fluid injected should be about 10 times lower than that of the fluid into which it is injected," said Juanes. "If the contrast is greater than 10, then the injection should be done more slowly to achieve the fastest maximum mixing. Otherwise, the low viscosity fluid will create a single channel through the thicker fluid, which is not ideal."

Cueto-Felgueroso said a similar process is at work in the engraved channels of a microfluidic device and in subsurface rock containing oil. "Mixing fluids at small scales or velocities is difficult because you can't rely on turbulence: it would be hard to stir milk into your coffee if you were using a microscopic cup," Cueto-Felgueroso said. "With viscous fingering, you let the fluids do the job of stirring."

This work was funded by the Italian energy company, Eni, and the ARCO Chair in Energy Studies.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Massachusetts Institute of Technology, Department of Civil and Environmental Engineering. The original article was written by Denise Brehm.

Journal Reference:

Birendra Jha, Luis Cueto-Felgueroso, Ruben Juanes. Fluid Mixing from Viscous Fingering. Physical Review Letters, 2011; 106 (19) DOI: 10.1103/PhysRevLett.106.194502

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here