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.

Morphologically driven dynamic wickability is essential for determining the hydrodynamic status of solid-liquid interface. We demonstrate that the dynamic wicking can play an integral role in supplying and propagating liquid through the interface, and govern the critical heat flux (CHF) against surface dry-out during boiling heat transfer. For the quantitative control of wicking, we manipulate the characteristic lengths of hexagonally arranged nanopillars within sub-micron range through nanosphere lithography combined with top-down metal-assisted chemical etching. Strong hemi-wicking over the manipulated interface (i.e., wicking coefficients) of 1.28 mm/s^0.5 leads to 164% improvement of CHF compared to no wicking. As a theoretical guideline, our wickability-CHF model can make a perfect agreement with improved CHF, which cannot be predicted by the classic models pertaining to just wettability and roughness effects, independently.

In this review, we cover recent advances in microfluidic colloid separation enabled by diffusiophoresis. Diffusiophoresis describes the motion of colloidal particles induced by local chemical gradients. Despite its long history, it is only recently that diffusiophoresis has gained a renewed interest in the scientific community. Such a resurgence is, in part, due to the recognition that diffusiophoresis may enable useful applications that are otherwise difficult to achieve or can be augmented by it. One example is continuous colloid separation, which requires the control of particle motion relative to the flow. This review aims to provide a fundamental understanding of diffusiophoretic separation along with a survey of the recent literature on diffusiophoretic colloid separation in microfluidic flows in both cross-gradient and counter-gradient settings.

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 show 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.

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.