Concentration gradients of amphiphiles are encountered in many biological (bacterial chemotaxis) and synthetic systems (applications of consumer products). This presentation will describe an experimental investigation that explores the use of thin films of liquid crystals (LCs) to report and quantify surfactant concentration gradients as well as to provide insight into the fundamental dynamic properties of amphiphiles at interfaces. We use a microfluidic system to generate controlled gradients in surfactant concentration and characterize the optical response of films of LCs exposed to those gradients. We find that the imposed surfactant concentration gradients trigger position-dependent orientational transitions in the LC films that lead to striking optical responses consistent with realignment of the LCs. Results will be presented to demonstrate that the optical responses reflect interfacial tension gradients generated by the surfactants (Marangoni flow-induced reorganization of LC) which leads to internal circulating flows within the LC films.  These effects lead to optical responses in the LC films that correlate closely with the magnitudes of the surfactant concentration gradients in bulk aqueous solution.  Additionally, by varying the lateral size of the LC domains, we demonstrate that the spatial extent of the optical response of the LC can be an order of magnitude larger than the characteristic width of the bulk surfactant concentration gradient. This result will be discussed in terms of the characteristic times for interfacial advection and adsorption/desorption of surfactant from the LC-aqueous interface. Overall, our results hint at new principles for direct optical reporting of gradients in solute concentration in ways that have the potential to be useful in emerging technologies such as soft robotics (to guide chemotaxis).

Optical response of liquid crystals to external gradients in surfactant concentration

ACS Fall 2021

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