ACMD Seminar: Interfacial Flows Driven by Solute Concentration Gradients in Complex Fluids

Brian McKenzie
Ph.D. Candidate, Dept. of Chemical Engineering, Carnegie Mellon University

Thursday, Nov. 14, 2024, 3:00-4:00 PM ET (1:00-2:00 PM MT)

Abstract: In natural and technological contexts, concentration gradients of solute molecules often arise by accident or by design. In the presence of a solid or fluid interface, these concentration gradients may drive fluid motion through phenomena including diffusio-osmotic flow and the Marangoni effect. Diffusio-osmotic flow is driven by solute–interface interactive forces distributed across a diffuse interfacial boundary layer, leading to sharp (nanoscale) velocity gradients when tangential concentration gradients are introduced. Diffusio-osmotic flows on the surface of a colloidal particle are responsible for diffusiophoresis, the deterministic migration of a particle along a solute concentration gradient. Marangoni stresses refer to tangential gradients in interfacial tension e.g. due to adsorption of surfactant molecules, driving motion at fluid–fluid interfaces.

These processes can be impactful on mass transfer of colloids and multiphase fluids: for example, diffusiophoresis has the potential to deliver particles at rates far greater than colloidal diffusion, and Marangoni stresses can generate convective instabilities that disrupt the fluid–fluid interface, in some cases even driving spontaneous emulsification of one phase into the other.

Existing research on these topics deal primarily with simple solutes, neglecting self-assembly processes such as micellization, complexation, and formation of vesicles. Here, I show that such processes can often strengthen or even qualitatively change the mass transport driven by interfacial flows. First, I will present an analytical calculation of the asymptotic deformations to a viscous fluid drop undergoing diffusiophoresis. Second, I will discuss our quantification of colloidal diffusiophoresis in the presence of micellizing ionic surfactants and surfactant–polymer complexes, using a numerical transport model to identify signatures of key physics in our microfluidic experiment. Lastly, I will discuss a novel hydrodynamic instability driven by adsorption and association of cationic and anionic surfactants meeting at a liquid–liquid interface, with a focus on explaining the instability mechanism through quasi-steady stability analysis and numerical simulation of the coupled chemical transport, reaction kinetics, and fluid mechanics.

Bio: Brian McKenzie is a Ph.D. Candidate in the Department of Chemical Engineering at Carnegie Mellon University, whose research uses mathematical techniques including asymptotic analysis, perturbation methods, and numerical modeling to explain and quantify non-intuitive phenomena in interfacial and colloidal fluid mechanics. Before starting doctoral research, he obtained undergraduate degrees in Mathematics and Chemical Engineering at the University of Florida, where his undergraduate research analyzed instability and bifurcation of pendant fluid drops.

Host: Leroy Jia

Note: This talk will be recorded to provide access to NIST staff and associates who could not be present to the time of the seminar. The recording will be made available in the Math channel on NISTube, which is accessible only on the NIST internal network. This recording could be released to the public through a Freedom of Information Act (FOIA) request. Do not discuss or visually present any sensitive (CUI/PII/BII) material. Ensure that no inappropriate material or any minors are contained within the background of any recording. (To facilitate this, we request that cameras of attendees are muted except when asking questions.)

*Safety Precaution: The hallway leading from the Courtyard to the exit closest to B-111 and B-113 will be used by contractors to move debris, machinery, and other supplies, as well as will be heavily trafficked by the contractors throughout the process. Be aware of the safety precautions posted during this time.

Note: Visitors from outside NIST must contact Meliza Lane at least 24 hours in advance.