Intellectual Merit: The design of nanoengines that can convert stored chemical energy into motion is a key transformative challenge of nanotechnology, especially for nano-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 responsible for this motion is not known, although a number of ideas have been put forth. This project involves a very simple mechanism is proposed: osmotic propulsion. A surface chemical reaction creates local concentration gradients of the reactive and product species which generate a net osmotic force on the motor. The motor is able to harness the ever present random thermal motion via a chemical reaction to achieve directed autonomous motion. This research demonstrates that such an 'osmotic' motor is possible and addresses such questions as: How fast can the motor move? How large of a force can it generate? How much 'cargo' can it carry? How much fluid can it pump? How can its motion be controlled and directed? What chemistry can be used? What is the efficiency of such an osmotic motor?
Broader Impact: Osmotic propulsion provides a very simple and general means to convert chemical energy into mechanical motion and work. Exploiting the random thermal motion in colloidal systems via osmotic propulsion can revolutionize the design and operation of microfluidic and nanodevices, with applications in directed self-assembly of materials, thermal management of micro- and nanoprocessors, and the design and operation of chemical and biological sensors. This research will provide explicit prescriptions for the construction and operation of colloidal particles that can be used as osmotic motors. This fundamental and transformative study must be undertaken if we wish to enable many of the nano-scale technologies envisioned for the future: tiny medical 'nanobots' that can access human illness inside the body, at the cellular level, and repair it. Or devices that can sense their way through micro channels in 'lab on a chip' devices, stirring or separating nano-liters of chemicals. Or even a nano-motor that senses intrusion of a specific molecule, swims toward it, and closes a channel in the process triggering an alarm switch for biological contaminants. Any of these types of devices is possible provided the physics of motion at that scale is correctly understood and utilized. And finally, studies of autonomous motors may help to understand more generally chemomechanical transduction as occurs in biological systems, and also create novel artificial motors that mimic living organisms and which can be harnessed to perform useful tasks.
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) not known, although a number of ideas have been put forth. In this research a very simple mechanism was proposed and studied. A chemical reaction occurring at the surface of a motor creates local concentration gradients of both the reactant (the fuel) and the product species. As these species diffuse in an attempt to re-establish equilibrium, they entrain the motor causing it to move. This process can be viewed either as osmotic propulsion or as self-diffusiophoresis. Net motion of the motor can occur if there is asymmetry in the reactivity of the motor surface, for example, by coating half of the surface with a catalyst. The concentration distributions of reactants and products generated by the surface reaction are determined by the motor size, the extent of the asymmetry in the reactivity, the speed of the chemical reaction and the diffusion coefficients of the reactants and/or products. For slow reactions the reaction speed determines the self-propulsion. When surface reaction dominates over diffusion the motor velocity cannot exceed the diffusive speed of the reactants, which is given by the reactant diffusivity divided by the motor size. The predictions for the motor speed as a function of motor size, diffusivities, stoichiometry of the chemical reaction and reaction speed showed excellent agreement with Brownian Dynamics simulations. The predicted magnitudes are in agreement with the experimental observations. The theoretical development shows that what is essential for propulsion is breaking the symmetry of the motor. This can be achieved via asymmetric surface reactivity, but it also can be achieved by controlling the shape of the motor, which may lead to easier fabrication of nanomotors; motors of prescribed shape can be uniformly coated with catalyst in a subsequent processing step. The theory also suggests that motors confined to an interface between two fluids can propelled by the same mechanism. This research demonstrates that osmotic propulsion is possible and addresses such questions as: How fast can the motor move? How large of a force can it generate? How much ‘cargo’ can it carry? How much fluid can it pump? What chemistries can be used for propulsion? And how efficient is such a motor? Through the mechanisms proposed in this research, a catalytic nanomotor is able to harness the ever-present random thermal motion via a chemical reaction to achieve directed autonomous motion.
