D.B. Robinson Distinguished Speaker Series 2013-2014
ICI Lecture Part 2:
Osmotic Propulsion: The Osmotic Motor
The design of nanoengines that can convert stored chemical energy into motion is a key challenge of nanotechnology, especially for engines that can operate autonomously. Recent experiments have demonstrated that it is possible to power the motion of nanoscale and microscale objects by using surface catalytic reactions – so-called catalytic nanomotors. The precise mechanism(s) responsible for this motion is(are) still debated, although a number of ideas have been put forth. Here, a very simple mechanism is discussed: osmotic propulsion.
A surface chemical reaction creates local concentration gradients of the reactant (the fuel) and product species. As these species diffuse in an attempt to re-establish equilibrium, they generate an osmotic force on the motor causing it to move. The concentration distributions are governed by the ratio of the surface reaction velocity and the diffusion velocity of the reactants. For slow reactions the reaction velocity determines the self-propulsion. When surface reaction dominates over diffusion the motor velocity cannot exceed the diffusive speed of the reactants. The theoretical predictions are compared with experiment and Brownian Dynamics simulations. The mathematical description of osmotic propulsion is analogous to that describing the swimming of microorganisms at low Reynolds numbers, leading to the alternate name chemical swimming. It is also shown that symmetry can be broken by the motor shape and, even for uniform reactivity, propulsion can be achieved – chemical sailing. Finally, the interactions of multiple motors can give rise to chemical swarming. Through osmotic propulsion a motor is able to harness the ever-present random thermal motion via a chemical reaction to achieve autonomous motion.
John F. Brady is the Chevron Professor of Chemical Engineering and Professor of Mechanical Engineering at the California Institute of Technology. He received his BS in chemical engineering from the University of Pennsylvania in 1975, which was followed by a year at Cambridge University as a Churchill Scholar. He received both an MS and PhD in chemical engineering from Stanford University, the latter in 1981. Following a postdoctoral year in Paris at ESPCI, he joined the Chemical Engineering department at MIT. Dr. Brady moved to Caltech in 1985.
Dr. Brady’s research interests are in the mechanical and transport properties of two-phase materials, especially complex fluids such as biological liquids, colloid dispersions, suspensions, porous media, etc. His research combines statistical and continuum mechanics to understand how macroscopic behavior emerges from microscale physics. He is the co-inventor of the Stokesian Dynamics technique for simulating the behavior of particles dispersed in a viscous fluid under a wide range of conditions.
Dr. Brady has been recognized for his work by several awards, including a Presidential Young Investigator Award, the Professional Progress Award of the American Institute of Chemical Engineers, the Bingham Medal of the Society of Rheology and the Fluid Dynamics Prize of the American Physical Society, Division of Fluid Dynamics. Dr. Brady served as an associate editor of the Journal of Fluid Mechanics and editor of the Journal of Rheology. He is a fellow of the American Physical Society and a member of the National Academy of Engineering.