Self-propelled fluids

Dr. Neil Canter, Contributing Editor | TLT Tech Beat July 2017

Researchers have taken key components from biological systems and prepared a simplified system that moves by itself.

KEY CONCEPTS
• A simplified system taking key components from biological systems is used to demonstrate self-propelled fluid flow for the first time in a non-biological system.
• The self-propulsion involves moving microtubules through the use of a naturally derived material known as kinesin.
• The fuel used to drive the process is ATP that is replenished by using an enzymatic regeneration system.

FLUID FLOW IS AN IMPORTANT FEATURE because lubricants need to move in some cases through narrow gaps between sliding surfaces to ensure that any friction created between the two surfaces is minimized. This characteristic also is important with hydraulic fluids that transmit mechanical energy produced by a pump.

Development of micropumps to facilitate the movement of hydraulic fluids at the microscale has been a challenge because such a device might be used in medical applications and must be prepared with biocompatible materials. In a previous TLT article, a gas permeation micropump prepared from polydimethylsiloxane and containing three distinct layers is able to pump fluids at a rate of 200 nanoliters of fluid per minute (1). Fluids used with the micropump included aqueous solutions of glucose, proteins and blood. 

But suppose a hydraulic fluid could be developed that can flow by itself or be self-propelled? Seth Fraden, professor of physics at Brandeis University in Waltham, Mass., says, “In conventional fluid flow, an external source must input energy to facilitate the movement of fluid. One example is the use of gravity to enable water to flow downhill. But biological systems can sustain fluid flow by themselves because the energy for propulsion is in the fluid itself.”

Fraden characterizes this self-propulsion as occurring in a hierarchical manner where molecules in a biological system can act as molecular motors that push on the main component in the fluid, water forcing the fluid to flow. This is in contrast to conventional ways of producing fluid motion where energy is fed in from outside the system in the form of pistons or propellers. 

The source of the chemical energy in biological systems is adenosine triphosphate (ATP). Such autonomous flow also known as self-organized coherent flow has been found in the early stage development of some organisms. Fraden says, “In embryology, as cells start to divide during growth, rapid mixing occurs through a process known as cytoplasmic streaming that stirs fluids faster than diffusion. This process is needed to ensure that specific organs develop in the proper location.”

Research now has been done to demonstrate self-organized coherent fluid flow in a non-biological system for the first time.

MICROTUBULE-KINESIN-ATP REACTION
The team led by Fraden and his colleague, Zvonimir Dogic, have taken key components from biological systems and prepared a simplified system that will move by itself. The key components are microtubules extracted from a cow’s brain, kinesin and depleting polymer. 

Fraden says, “The microtubules were produced through polymerization just above room temperature followed by annealing at room temperature and had an average length of approximately one micron. Kinesin is prepared from a 401-amino acid N-terminal domain that was extracted from the insect Drosophila melanogaster. The depleting polymer is a commercially available ethylene oxide, propylene oxide block copolymer that assembles into approximately 20 nanometer diameter micelles.”

The researchers incorporated these components in an aqueous solution that was placed in a vessel prepared with a cyclic olefin copolymer substrate covered with a film made from the same substrate. Added to the aqueous solution was ATP, and to replenish the ATP the researchers included an enzymatic regeneration system.

Fraden says, “Self-propulsion started with the microtubules self-organizing at the boundary of the vessel, which can be considered to be a pipe. The microtubules act in a similar manner to a railroad track, which means that two of them line up in parallel. In the next step, a kinesin molecule will come together and link up with both microtubules in a similar manner to a railroad track tie. Kinesin is the engine that drives the microtubules by consuming ATP, which enables this material to push the microtubules in opposite directions. If one slides north, then the other slides south.”

With the start of mechanical motion, the microtubule-kinesin-ATP reaction breaks apart the structure and the kinesin then seeks out two more microtubule partners. The depleting polymer interacts with the fluid by forming an osmotic pressure gradient that forces fluid to flow while the polymer micelles bundle. 

Kinesin molecules will move approximately 15 nanometers from one microtubule to another and then start another microtubule-kinesin-ATP reaction. The net result is the formation of self-organized coherent flow that can continue as long as ATP is available to power the kinesin.

The researchers determined the rate of fluid motion by using tracer beads made from the dye, Alexa 488 impregnated in polystyrene particles. Fraden says, “The shape of the container is analogous to an inner tube of a tire. Microtubules push off the walls of the tube and propel the bulk of the fluid. Circular flow has been found to continue for up to 30 hours. Beyond that time, decomposition of the key components such as the microtubules and kinesin is anticipated because they are organic in nature. A recycling process is needed to extend the duration of fluid motion.”

One other factor is the geometry of the inner tube. Fraden says, “We can prepare systems that are up to four or five millimeters in diameter. But if the geometry of the cross-section is changed from circular to elliptical and the ratio of the long to the short side exceeds 3:1, self-propelled flow stops. We have no explanation for this phenomenon.”

Figure 1 shows the flow of the fluid confined in a cylinder. 


Figure 1. Confining active fluids in a cylinder triggers a transition from turbulent to coherent flowing states. The coherent flow coexists with vortices constantly created and dissipated. The criterion of such a transition is the aspect ratio of cylindrical height and radius ranging between a third and 3. In this case, the cylinder has a height of 1.3 millimeters and a width of 2.2 millimeters. Arrows represent flow field and the color represents their rotation. Blue and red tones represent clockwise and counterclockwise rotation, respectively. (Figure courtesy of Brandeis University.)

Future work will involve first determining how much work can be obtained from the self-organized coherent flow during use. One way to measure this is to determine how hard does the self-organized coherent flow push a rheometer.

A second aspect is to look at different curvatures (both positive and negative) to understand the forces enabling the self-organized coherent flow. As part of this process, local stress fields within the fluid also will be examined.

Additional information can be found in a recent article (2) or by contacting Fraden at fraden@brandeis.edu. 

REFERENCES
1. Canter, N. (2007), “Movement of fluids on the microscale,” TLT, 63 (4), pp. 12-13.
2. Wu, K., Hishamunda, J., Chen, D., DeCamp, S., Chang, Y., Nieves, A., Fraden, S. and Dogic, Z. (2017), “Transition from turbulent to coherent flows in confined three-dimensional active fluids,” Science, 355 (6331), eaal1979.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.