Jesse T. Ault

Size-dependent control of colloid transport via solute gradients in dead-end channels

Significance Dead-end geometries are commonly found in many porous systems. Particle transport into such dead-end pores is often important, but is difficult to achieve owing to the confinement. It is natural to expect that Brownian motion is the sole mechanism to deliver the particles into the pores, but that mechanism is, unfortunately, slow and inefficient. Here, we introduce solute gradient to control the transport of particles in dead-end channels. We demonstrate a size effect such that larger particles tend to focus more and reside deeper in the channels. Our findings suggest a potential pathway to many useful applications that are difficult to achieve in dead-end geometries such as particle sorting and sample preconcentration, which are important in pharmaceuticals and healthcare industries. Transport of colloids in dead-end channels is involved in widespread applications including drug delivery and underground oil and gas recovery. In such geometries, Brownian motion may be considered as the sole mechanism that enables transport of colloidal particles into or out of the channels, but it is, unfortunately, an extremely inefficient transport mechanism for microscale particles. Here, we explore the possibility of diffusiophoresis as a means to control the colloid transport in dead-end channels by introducing a solute gradient. We demonstrate that the transport of colloidal particles into the dead-end channels can be either enhanced or completely prevented via diffusiophoresis. In addition, we show that size-dependent diffusiophoretic transport of particles can be achieved by considering a finite Debye layer thickness effect, which is commonly ignored. A combination of diffusiophoresis and Brownian motion leads to a strong size-dependent focusing effect such that the larger particles tend to concentrate more and reside deeper in the channel. Our findings have implications for all manners of controlled release processes, especially for site-specific delivery systems where localized targeting of particles with minimal dispersion to the nontarget area is essential.

Vortex-Breakdown-Induced Particle Capture in Branching Junctions.

We show experimentally that a flow-induced, Reynolds number-dependent particle-capture mechanism in branching junctions can be enhanced or eliminated by varying the junction angle. In addition, numerical simulations are used to show that the features responsible for this capture have the signatures of classical vortex breakdown, including an approach flow aligned with the vortex axis and a pocket of subcriticality. We show how these recirculation regions originate and evolve and suggest a physical mechanism for their formation. Furthermore, comparing experiments and numerical simulations, the presence of vortex breakdown is found to be an excellent predictor of particle capture. These results inform the design of systems in which suspended particle accumulation can be eliminated or maximized.

Low‐Cost Zeta Potentiometry Using Solute Gradients

The zeta potential is an electric potential in the Debye screening layer of an electrolyte, which represents a key physicochemical surface property in various fields ranging from electrochemistry to pharmaceuticals. Thus, characterizing the zeta potential is essential for many applications, but available measurement techniques are limited. Electrophoretic light scattering is typically used to measure the zeta potential of particles in suspension, whereas zeta potential measurements of a solid wall in solution rely on either streaming potential or electroosmotic mobility measurement techniques, both of which are expensive and sophisticated. Here, a simple, robust method to simultaneously measure the zeta potential of particles in suspension and solid walls is presented. The method uses solute gradients to induce particle and fluid motions via diffusiophoresis and diffusioosmosis, respectively, which are both sensitive to the zeta potential of the particle and the wall. By visualizing the particle dynamics, both zeta potentials can be determined independently. Finally, a compact microscope is used to demonstrate low-cost zeta potentiometry that allows measurement of both particle and wall zeta potentials, which suggests a cost-effective tool for pharmaceuticals as well as for educational purposes.

Flow-Driven Rapid Vesicle Fusion via Vortex Trapping.

Fusion between suspended lipid vesicles is difficult to achieve without membrane proteins or ions because the vesicles have extremely low equilibrium membrane tension and high poration energy. Nonetheless, vesicle fusion in the absence of mediators can also be achieved by mechanical forcing that is strong enough to induce membrane poration. Here, we employ a strong fluid shear stress to achieve vesicle fusion. By utilizing a unique vortex formation phenomenon in branched channels as a platform for capturing, stressing, and fusing the lipid vesicles, we directly visualize using high-speed imaging the vesicle fusion events, induced solely by shear, on the time scale of submilliseconds. We show that a large vesicle with a size of up to ∼10 μm can be achieved by the fusion of nanoscale vesicles. This technique has the potential to be utilized as a fast and simple way to produce giant unilamellar vesicles and to serve as a platform for visualizing vesicle interactions and fusions in the presence of shear.

Point-source imbibition into dry aqueous foams

We use experiments, modeling and numerics to study the imbibition dynamics from a point source into a homogeneous dry aqueous foam. A distinctive feature of foams compared to solid porous material is that imbibition occurs in the liquid microchannels of the foam called Plateau borders, which have a volume varying in space and time. Dynamics is driven by the capillary pressure and resisted by the viscous and gravity forces in the liquid microchannels. Assuming a constant pressure in the imbibing liquid reservoir, we show that the imbibition front advances and flattens out in time due to gravity, the effect of which is quantified by introducing the Bond number B, which compares the gravitational effects to the capillary pressure using the mean bubble radius as the characteristic length. This evolution describes both miscible and immiscible imbibing liquids. For the latter, we introduce the idea of an effective interfacial tension to take the oil-water interfacial energy into account. The details of the imbibition process are confirmed by experiments and numerics using foams with tangentially immobile interfaces in the channel-dominated model.