Non-equilibrium colloid dynamics

Size-dependent targeted delivery in dead-end pores 
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. 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. 

Publications:
Shin et al., PNAS 113 257 (2016); Ault et al., in preparation

Membraneless water filtration using CO2
Water purification technologies such as microfiltration/ultrafiltration and reverse osmosis utilize porous membranes to remove suspended particles and solutes. These membranes, however, cause many drawbacks such as a high pumping cost and a need for periodic replacement due to fouling. Here we present an alternative membraneless method for separating suspended particles by exposing the colloidal suspension to CO2. Dissolution of CO2 into the suspension creates solute gradients that drive phoretic motion of particles. Due to the large diffusion potential generated by the dissociation of carbonic acid, colloidal particles move either away from or towards the gas-liquid interface depending on their surface charge. Using the directed motion of particles induced by exposure to CO2, we demonstrate a scalable, continuous flow, membraneless particle filtration process that exhibits low energy consumption, three orders of magnitude lower than conventional microfiltration/ultrafiltration processes, and is essentially free from fouling. 

​Publications:
Shin et al., Nat. Commun., 8 15181 (2017); Shardt et al., in preparation

Low-cost zeta potentiometry using solute gradients
In contact with an electrolyte solution, a particle or pore surface typically acquires a surface charge. Experimentally the charge state of the surface is usually characterized by the zeta potential, which is thus a key physicochemical surface property in fields ranging from electrochemistry and electrokinetics to colloid science and phamaceuticals. Measuring the zeta potential is essential for many applications, but available techniques are expensive and unwieldy. For example electrophoretic light scattering is 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. We present an alternative, simple, robust method to simultaneously measure the zeta potential of colloidal particles in suspension, and of the solid walls of the containing capillary. The method uses solute gradients to induce particle and fluid motions via diffusiophoresis and diffusioosmosis, respectively. By visualizing the particle dynamics, both zeta potentials can be determined independently.  We further show how measurements can be made with inexpensive equipment such as a commercial USB microscope, thus opening up low-cost zeta potentiometry as a cost-effective tool for pharmaceutical as well as for educational purposes.  

Publications:
​Shin et al., Adv. Mater., 29 1701516 (2017)