We are designing, synthesizing and exploring the properties of surfactants that can be turned “on” and “off” in solution.
Figure 1 shows the molecular structures of some surfactants synthesized in our laboratory. These surfactants can be switched between states by using electrical potentials (top left), illumination (middle right) and specific chemical environments (bottom left).
The properties of aqueous solutions of these surfactants differ substantially in each of the two states of the surfactants. These surfactants permit the properties of surfactant solutions to be cycled and to be changed with a spatial resolution of micrometers. For example, Figure 2 shows the motions of small droplets of oil that are being propelled around some fluidic channels by using surfactants to create controlled gradients in surface tension. The gradient is created by turning the surfactant “on” at one end of the channel and “off” at the other end.
Alternatively, Figure 3 shows the patterned de-wetting of a film of aqueous solution that is driven by a spatially-localized change in the state of a surfactant. A continuous film of liquid (top image) breaks up into an array of droplets (bottom image). The image shown is an aerial view.
It is also possible to use these types of surfactants to release droplets of liquid from an array of capillaries by selectively illuminating the capillaries with light. This phenomenon is shown in Figure 4.
We believe that principles for active control of surfactants may find use in micro-scale chemical process systems where interfacial effects tend to dominate the behavior of fluids. For example, gradients in surface tension can be used to drive transport processes at interfaces and in the bulk of micro-scale reactors. Alternatively, micro-scale separations processes may be based on reversible assembly and disassembly of micelles. We are working to demonstrate a micro-scale analytical system for detection of a pesticide commonly used in the US.
We are also using molecular dynamics simulations to guide the design of future generations of these types of tunable surfactants. We are particularly interested in understanding how the molecular structure of a surfactant influences its dynamics properties. This relationship is poorly understood in surfactant science, in general. As shown in Figure 5, we have used molecular dynamics to evaluate the potential-of-mean force for the adsorption of a simple surfactant (decanol) to the surface of water. We are able to use the potential-of-mean force to calculate the kinetic constants for adsorption and desorption of the surfactant. Surprisingly good agreement is found with experimental estimates.