CAREER: Decoding the dynamics of complex fluids near surfaces and interfaces

When liquid drops contact a solid surface, they can wet the surface and spread over it, which is important in a variety of industrial and technological processes such as cooling industrial reactors, applying pesticides to the leaves of plants, oil recovery from underground reservoirs, and inkjet printing. Understanding wetting and evaporation of biological fluids on solid surfaces is a critical step toward solving problems related to human health, such as preventing disease transmission by virus-loaded droplets.

Wetting, spreading and evaporation of liquid drops can be strongly affected by the presence of surface-active agents in the liquid, such as surfactants or nanoparticles, as well as the properties of the solid surface. Predicting and controlling spreading of liquids is challenging in this case and formulations often are based on trial-and-error experiments.

This CAREER project will investigate how the composition of liquids, including simple liquids, liquids containing surfactants and/or nanoparticles, and biological fluids, affect droplet wetting, spreading, and evaporation. A series of experiments will be carried out on droplets of various composition to reveal how various components in the liquid contribute individually and synergistically to the physics of wetting and spreading dynamics on a series of solid surfaces.

The results will identify key parameters in wetting, spreading and evaporation, and also uncover details of complex flows at interfaces that affect these processes. The research will be used in training a diverse group of undergraduate and graduate students. It will also enable the creation of educational videos and demonstrations to engage middle school and high school students in scientific activities, attract female veterans to science and inspire them to pursue higher education as they transition to civilian life, and enhance the science literacy of the public.

A large body of literature is available on the wetting, spreading, and evaporation of simple fluids; however, concepts established for simple fluids cannot be directly applied to droplets of complex fluids carrying surfactants and nanoparticles. The understanding of key factors in spreading and evaporation in a mixed particle/surfactant system and the impact of particle/surfactant synergism in these cases is limited.

The scientific goal of this CAREER project is to advance fundamental understanding of the wetting and spreading in case of multicomponent fluidic systems by elucidating the roles that colloidal particle interfacial adsorption and solid surface energies play in the dynamics. Surface properties of the particles will be modified to alter the energy barrier for nanoparticle adsorption to the fluid/fluid interface.

Magnitudes of the polar and dispersive components of the solid surface energy will be systematically tuned to examine the role of interaction forces between the complex fluid and the substrate in the wetting behavior. These studies will identify the influence of nanoparticle interfacial adsorption, and potentially the interfacial rheology of the particle layer, on spreading dynamics and evaporation kinetics of a sessile drop.

Mixed particle/surfactant dispersions will be examined to illustrate whether the interfacial confinement of nanoparticles combined with Marangoni effects influences wetting behavior and whether interfacial rheology affects drying in multicomponent systems. Artificial saliva models will be used as a base fluid to investigate the importance of mucin at the fluid-fluid interface on spreading and evaporation on various surfaces.

This research addresses the need to develop an empiricism-free understanding of the behavior of complex fluids near surfaces and interfaces, and the function of surface characteristics in altering wettability in such systems.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.