Solute-mediated colloidal vortex in a microfluidic T-junction

Abstract: Solute gradients next to an interface drive a diffusioosmotic flow, the origin of which lies in the intermolecular interactions between the solute and the interface. These flows on the surface of colloids introduce an effective slip velocity, driving their diffusiophoretic migration. In confined environments, we expect the interplay between diffusiophoretic migration and diffusioosmotic flows near the walls to govern the motion of colloids. These near-wall osmotic flows are, however, often considered weak and neglected. Here, using microfluidic experiments in a T-junction, numerical simulations, and theoretical modeling, we show that the interplay between osmotic and phoretic effects leads to unexpected outcomes: forming a colloidal vortex in the absence of inertial effects, and demixing and focusing of the colloids in the direction opposite to what is commonly expected from diffusiophoresis alone. We show these colloidal vortices to be persistent for a range of salt types, salt gradients, and flow rates, and establish a criterion for their emergence. Our work sheds light on how boundaries modulate the solute-mediated transport of colloids in confined environments.

• The paper demonstrates that solute gradients can induce persistent colloidal vortices and unexpected particle focusing in microfluidic T-junctions, even without inertial forces.
• The study reveals a key quantitative relationship, showing the colloidal focusing line scales proportionally to $ extrm{Pe}^{-1/3}$ and depends on diffusioosmotic and diffusiophoretic mobilities.
• These findings have significant implications for applications like microfluidic particle sorting, drug delivery, and understanding solute-mediated transport in complex biological and geological environments.

Solute-mediated Colloidal Vortex in a Microfluidic T-junction

This study explores the intricate dynamics of solute-mediated transport phenomena, emphasizing the formation of colloidal vortices within the confines of a microfluidic T-junction. It addresses the foundational interplay between diffusioosmotic flow and diffusiophoretic migration and identifies unexpected directional migration and particle accumulation phenomena induced by solute gradients. Using a robust experimental framework complemented by numerical simulations and theoretical modeling, the authors elucidate the formative conditions under which colloidal vortices arise.

The authors leverage microfluidic experiments to reveal the presence of persistent colloidal vortices that emerge in the absence of inertial forces. These phenomena radically diverge from conventional diffusiophoretic expectations. The study underscores the critical role of solute-induced velocity fields on particle dynamics, often overshadowed by the variable interplay of diffusiophoresis (particle motion induced by solute concentration gradients) and diffusioosmosis (fluid flow driven by solute gradients along surfaces). The research demonstrates that even without significant inertia, colloidal vortices can persist across a variety of salt types, concentration gradients, and flow conditions.

In a confined T-junction, colloids typically migrate due to an effective slip velocity imparted by solute gradients. This interplay and its subsequent influence on colloidal motion form the cornerstone of the findings. Contrary to the assumption that near-wall osmotic flows are negligible, the study provides evidence of significantly influential near-wall diffusioosmotic flows that modify expected particle trajectories. These findings illustrate an unexpected migration and focusing of colloids, a phenomena counterintuitive to predictions solely based on diffusiophoretic migration, where particles would align along solute concentrations.

Key quantitative insights were gained by determining the scaling relationships governing the colloidal focusing line, denoted as $x_f$, which scales proportionally to $\textrm{Pe}^{-1/3}$. This dependence highlights the interaction between Peclet number (Pe) and channel geometry (height and flow velocity), revealing that the focusing line's location is modulated by the diffusioosmotic and diffusiophoretic mobility coefficients, $\Gamma_w$ and $\Gamma_p$ respectively. The emergence of the vortex is associated with a net compressibility in colloidal motion, as particles amass within focused regions attributed to effective sink-like dynamics, contributing significantly to non-fickian transport behavior in such confined domains.

The implications extend to practical applications such as controlled particle sorting in microfluidic devices and broader relevance in understanding solute-mediated transport in complex mediums, ranging from biofluidic environments to porous geological formations. The discovery of these vortices sheds light on potential mechanisms behind solute-driven transport and segregation within biological systems, implicating roles in cellular and subcellular transport influenced by solute gradients.

In future developments, exploring the variations of surface charge or material composition as controls for diffusioosmotic mobility offers intriguing possibilities for modulating such phenomena purposefully. Furthermore, the interplay of mobility factors and geometry within different media could guide advancements in industrial applications such as enhanced oil recovery or targeted drug delivery. Given the fundamental insights provided by the interplay of phoretic and osmotic forces, the observation of unexpected colloidal motion in solute gradients paves the way for innovative explorations in both synthetic systems and natural environments.