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Reports: ND554141-ND5: Suspension Interactions via Gradients Generated in situ: Breaking Emulsions and Triggering Flocculation

Todd M. Squires, University of California (Santa Barbara)

The object of this project is to demonstrate, harness, and engineer long-ranged, non-equilibrium but long-lasting interactions between particles in suspension. The broad approach exploits diffusiophoresis, which is a phoretic migration that has long been known to occur for colloidal particles suspended in solutions with chemical gradients. We seek to demonstrate "soluto-inertial," interactions, in which one object establishes a long-lived solute or solvent flux, which subsequently drives other suspended particles into diffusiophoretic migration. Given the range of solutes, solvents, and particle chemistries available synthetically and naturally, we ultimately envision a broad class of tools to manipulate and control suspensions.

It has been known that particles generally migrate diffusiophoretically when suspended in a solution gradient; however, it has remained challenging even to visualize diffusiophoretic migration directly, much less to measure the broad set of diffusiophoretic mobilities of different particles in solute/solvent gradients of different concentration and magnitude that will be required for our project.

To this end, we had previously developed methods to design and impose controllable solute/solvent gradients in a microfluidic system, wherein photopolymerized hydrogel membranes act as microdialysis membranes (fig. 1). Fig. 1. (a) Microfluidic device to impose chemical gradients of defined concentration and magnitude. (b) Hydrogel microdialysis membranes, photopolymerized in situ, allow diffusive exchange between reservoir channels (A/C) and sample gradient channel (B).

Text Box: Fig. 1. (a) Microfluidic device to impose chemical gradients of defined concentration and magnitude. (b) Hydrogel microdialysis membranes, photopolymerized in situ, allow diffusive exchange between reservoir channels (A/C) and sample gradient channel (B).

Using this system, we have measured the diffusiophoretic migration of various species, including fluorescent polystyrene particles, under gradients of the ionic surfactant sodium dodecylsulfate (SDS), as a function of concentration and gradient strength. We observe that polystyrene colloids migrate down SDS gradients diffusiophoretically (fig. 2), have developed a theory for this migration, and are working to compare theoretical predictions with experimental measurements. Fig. 2. Fluorescent polystyrene particles migrate diffusiophoretically down SDS gradients established within the microfluidic device shown in fig. 1.

Text Box: Fig. 2. Fluorescent polystyrene particles migrate diffusiophoretically down SDS gradients established within the microfluidic device shown in fig. 1.

Fig.3. Soluto-inertial post (150um radius) establishes and maintains an SDS outflux into a water+PS colloid suspension. Since PS particles migrate diffusiophoretically down SDS gradients (fig. 2), PS particles are driven away from the soluto-inertial post. A strong depletion zone forms around the post, and grows for tens of minutes. Even particles more than 100um away from the post experience a repulsive, non-equilibrium interaction.

Text Box: Fig. 3. Soluto-inertial post (150um radius) establishes and maintains an SDS outflux into a water+PS colloid suspension. Since PS particles migrate diffusiophoretically down SDS gradients (fig. 2), PS particles are driven away from the soluto-inertial post. A strong depletion zone forms around the post, and grows for tens of minutes. Even particles more than 100um away from the post experience a repulsive, non-equilibrium interaction.

In tandem, we have developed an experimental system in which a defined structure (fig. 3) establishes a long-lived outflux of SDS. Notably, the outflux lasts for tens of minutes, and extends hundreds of microns. PS particles suspended in the liquid around the structure migrate diffusiophoretically in response to the SDS gradient that is established by the structure. Experimental results are consistent with our hypothesized mechanism: PS particles indeed migrate down SDS gradients – meaning away from the structure – developing a dramatic void region around the structure that grows to hundreds of microns in extent, and continues to grow after tens of minutes.

In the near-term, we intend to quantitatively compare our theoretical models with experimental results, and thus to publish a compelling, proof-of-principle demonstration of this broad mechanism. Beyond that, we look to develop and demonstrate broad design rules for soluto-inertial structures and interactions, to establish the range of chemical specificities that can be targeted, and to tune the time- and length-scales over which such interactions are effective.