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., Soft Matter 13 9015 (2017)

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)

Colloidal Accumulation in Flow Junctions Induced by Fluid Flow and Diffusiophoresis
The flow of solutions containing solutes and colloidal particles in porous media is widely found in systems including underground aquifers, hydraulic fractures, estuarine or coastal habitats, water filtration systems, etc. In such systems, solute gradients occur when there is a local change in the solute concentration. While the effects of solute gradients have been found to be important for many applications, we observe an unexpected colloidal behavior in porous media driven by the combination of solute gradients and the fluid flow. When two flows with different solute concentrations are in contact near a junction, a sharp solute gradient is formed at the interface, which may allow strong diffusiophoresis of the particles directed against the flow. Consequently, the particles accumulate near the pore entrance, rapidly approaching the packing limit. These colloidal dynamics have important implications for the clogging of a porous medium, where particles that are orders of magnitude smaller than the pore width can accumulate and block the pores within a short period of time. We also show that this effect can be exploited as a useful tool for preconcentrating biomolecules for rapid bioassays

Publications:
​Shin et al., Phys. Rev. X, 7 041038 (2017); ​Ault et al., J. Fluid Mech., 854 420 (2018)

Cleaning by Surfactant Gradients: Particulate Removal from Porous Materials and the Significance of Rinsing in Laundry Detergency
Removing particles from fibrous materials involves loosening via surfactants followed by particle transfer in a flow. While flow advection is commonly believed to be the major driver for pore-scale transport, small pores within the fabric do not allow any significant fluid flow inside them, thus significantly reducing the role of advection. However, rinsing the fabric with fresh water naturally establishes a surfactant gradient within the pore space, providing a suitable environment for particles to undergo diffusiophoresis. We demonstrate that this mechanism can remove particles from deep within fabric pores at an accelerated rate. The nonlinear aspect of diffusiophoresis significantly prolongs the lifetime of the phoretic motion beyond the naive solute diffusion time scale during rinsing, allowing long-lasting, continuous removal of particles. Moreover, owing to the fine balance between chemiphoresis and electrophoresis for particles in anionic surfactant concentration gradients, we show that the particle removal is sensitive to the counterion mobility, suggesting a simple route to control the effect. We thus claim to have resolved the “stagnant core problem”—a long-standing mystery in laundry detergency—and have identified a physicochemical approach to particle transport in fibrous media with broad applicability.
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Publications:
​Shin et al., Phys. Rev. Appl, 9 034012 (2018); ​Warren et al., Soft Matter, 15 278 (2019)