Flow and rheology of interfaces at microscopic length scales

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0853837Weeks Viscoelastic interfaces are ubiquitous in nature; such as in cell membranes and in the alveoli of the lung, and can be artificially constructed; such as in emulsions/foams and soap films. Determining their rheology is, therefore, critical to our understanding of the dynamics of many processes that occur at these interfaces. Furthermore, this knowledge may help in the design of better materials for industrial applications, particularly in the food and cosmetics sector. In this project, the PIs will investigate the use of microscopic particles (diameters 20 nm - 2 microns) as probes of the viscoelasticity, and obtain the surface viscosity and elasticity by observing the thermal Brownian motion of the particles. Because of the size of the probes, this method is more sensitive than most interfacial rheology techniques that currently exist. By correlating the Brownian motion of pair of particles at different separations, they also will measure length-scale dependent viscoelasticity. This will relate the bulk properties of interfaces with their microstructure and dynamics, an approach that is lacking in other techniques. They plan to apply our method to: a) Systems with extremely low surface viscosity; specifically phospholipid molecules at an air water interface. Accurate measurements of surface viscosities below 10-8 Pa-s-m do not currently exist. b) Systems that have spatially heterogeneous domains, and therefore surface rheology that depends on the size of these domains. They will create these domains by applying surface pressure to phospholipids and fatty acids, and measure their length scale dependent rheology. c) Soap film hydrodynamics; the motion of a probe particle creates a flow field affecting the motion of other particles. Correlating the motion of particles by our technique, we will be able to measure this flow field. Further, The PI plans to actively perturb these interfaces by applying a known force to the probe particles. The drag experienced by the particles in response to the force is a direct measure of the viscoelasticity. By applying large forces to a particle, they will obtain the non linear response of the interface, which will be of fundamental importance and will have industrial relevance as well. Broad Impact: Viscoelastic interfaces are ubiquitous in nature; such as in cell membranes and in the alveoli of the lung, and can be artificially constructed; such as in emulsions/foams and soap films. Determining their rheology is therefore critical to our understanding of the dynamics of many processes that occur at these interfaces. Further, this may help in the design of better materials for industrial applications, particularly in the food and cosmetics sector. The PI will conduct at least one field trip each year, for groups of primary school students. We have done these in the past; these field trips give students hands on laboratory experiences. In particular, they plan to develop new activities around interfaces such as soap films (and foams in general). These field trips generally take everybody in our laboratory group one day of time (including preparation, interacting with the students, and cleaning up). The PI has an extensive commitment to undergraduate research, and has mentored 18 undergraduate researchers in our laboratory over the past five years. They plan to continue this by working with at least one undergraduate per year on this research, and possibly two per year (as often students work during the school year for academic credit). These undergraduates, and also the graduate student involved, will learn microscopy skills, computer data analysis, and some wet lab chemistry, providing useful experiences for whatever career directions they choose.