Diffusiophoresis, Diffusioosmosis, and Microfluidics: Surface-Flow-Driven Phenomena in the Presence of Flow.

Diffusiophoresis is the spontaneous motion of particles under a concentration gradient of solutes. Since the first recognition by Derjaguin and colleagues in 1947 in the form of capillary osmosis, the phenomenon has been broadly investigated theoretically and experimentally. Early studies were mostly theoretical and were largely interested in surface coating applications, which considered the directional transport of coating particles. In the past decade, advances in microfluidics enabled controlled demonstrations of diffusiophoresis of micro- and nanoparticles. The electrokinetic nature and the typical scales of interest of the phenomenon motivated various experimental studies using simple microfluidic configurations. In this review, I will discuss studies that report diffusiophoresis in microfluidic systems, with the focus on the fundamental aspects of the reported results. In particular, parameters and influences of diffusiophoresis and diffusioosmosis in microfluidic systems and their combinations are highlighted.

CO2-leakage-driven diffusiophoresis causes spontaneous accumulation of charged materials in channel flow.

We identify a phenomenon where the onset of channel flow creates an unexpected, charge-dependent accumulation of colloidal particles, which occurs in a common-flow configuration with gas-permeable walls, but in the absence of any installed source of gas. An aqueous suspension of either positively charged (amine-modified polystyrene; a-PS) or negatively charged (polystyrene; PS) particles that flowed into a polydimethylsiloxane (PDMS) channel created charge-dependent accumulation 2 to 4 min after the onset of flow. We unravel the phenomenon with systematic experiments under various conditions and model calculations considering permeability of the channel walls and [Formula: see text]-driven diffusiophoresis. We demonstrate that such spontaneous transport of particles is driven by the gas leakage through permeable walls, which is induced by the pressure difference between the channel and the ambient. Since the liquid pressure is higher, an outward flux of gas forms in the flow. We also observe the phenomenon in a bacterial suspension of Vibrio cholerae, where the fluorescent protein (mKO; monomeric Kusabira Orange) and bacterial cells show charge-dependent separation in a channel flow. Such experimental observations show that diffusiophoresis of charged particles in an aqueous suspension can be achieved by having gas leakage through permeable walls, without any preimposed ion-concentration gradient in the liquid phase. Our findings will help resolve unexpected challenges and biases in on-chip experiments involving particles and gas-permeable walls and help understand similar configurations that naturally exist in physiological systems, such as pulmonary capillaries. We also demonstrate potential applications, such as concentrating and collecting proteins below the isoelectric point.

CO2-Driven diffusiophoresis for maintaining a bacteria-free surface.

Dissolution and dissociation of CO2 in an aqueous phase induce diffusiophoretic motion of suspended particles with a nonzero surface charge. We report CO2-driven diffusiophoresis of colloidal particles and bacterial cells in a circular Hele-Shaw geometry. Combining experiments and model calculations, we identify the characteristic length and time scales of CO2-driven diffusiophoresis in relation to system dimensions and CO2 diffusivity. The motion of colloidal particles driven by a CO2 gradient is characterized by measuring the average velocities of particles as a function of distance from the CO2 sources. In the same geometrical configurations, we demonstrate that the directional migration of wild-type V. cholerae and a mutant lacking flagella, as well as S. aureus and P. aeruginosa, near a dissolving CO2 source is diffusiophoresis, not chemotaxis. Such a directional response of the cells to CO2 (or an ion) concentration gradient shows that diffusiophoresis of bacteria is achieved independent of cell shape, motility and the Gram stain (cell surface structure). Long-time experiments suggest potential applications for bacterial diffusiophoresis to cleaning systems or anti-biofouling surfaces, by reducing the population of the cells near CO2 sources.

Diffusiophoresis in Multivalent Electrolytes.

Diffusiophoresis is the spontaneous movement of colloidal particles in a concentration gradient of solutes. As a small-scale phenomenon that harnesses energy from concentration gradients, diffusiophoresis may prove useful for passively manipulating particles in lab-on-a-chip applications as well as configurations involving interfaces. Though naturally occurring ions are often multivalent, experimental studies of diffusiophoresis have been mostly limited to monovalent electrolytes. In this work, we investigate the motion of negatively charged polystyrene particles in one-dimensional salt gradients for a variety of multivalent electrolytes. We develop a one-dimensional model and obtain good agreement between our experimental and modeling results with no fitting parameters. Our results indicate that the ambipolar diffusivity, which is dependent on the valence combination of cations and anions, dictates the speed of the diffusiophoretic motion of the particles by controlling the time scale at which the electrolyte concentration evolves. In addition, the ion valences also modify the electrophoretic and chemiphoretic contributions to the diffusiophoretic mobility of the particles. Our results are applicable to systems where the chemical concentration gradient is comprised of multivalent ions, and motivate future research to manipulate particles by exploiting ion valence.

Diffusiophoresis: from dilute to concentrated electrolytes.

Electrolytic diffusiophoresis is the movement of colloidal particles in response to a concentration gradient of an electrolyte. The diffusiophoretic velocity vDP is typically predicted through the relation vDP = DDP ∇log cs, where DDP is the diffusiophoretic mobility and cs is the concentration of the electrolyte. The logarithmic dependence of vDP on cs may suggest that the strength of diffusiophoretic motion is insensitive to the magnitude of the electrolyte concentration. In this article, we emphasize that DDP is intimately coupled with cs for all electrolyte concentrations. For dilute electrolytes, the finite double layer thickness effects are significant such that DDP decreases with a decrease in cs. In contrast, for concentrated electrolytes, charge screening could result in a decrease in DDP with an increase in cs. Therefore, we predict a maximum in DDP with cs for moderate electrolyte concentrations. We also show that for typical colloids and electrolytes , where Ds is the solute ambipolar diffusivity. To validate our model, we conduct microfluidic experiments with a wide range of electrolyte concentrations. The experimental data also reveals a maximum in DDP with cs, in agreement with our predictions. Our results have important implications in the broad areas of electrokinetics, lab-on-a-chip, active colloidal transport and biophysics.

Diffusion of multiple electrolytes cannot be treated independently: model predictions with experimental validation.

We study the diffusion of multiple electrolytes in a one-dimensional pore. We model the scenario where an electrolyte is in contact with a reservoir of another electrolyte, such that the cation of the two electrolytes is common. The model reveals that several factors influence the ion concentration profiles: (i) relative diffusivities of the ions, (ii) ratio of the electrolyte concentrations in the pore and the reservoir, and (iii) the valence of the ions. We demonstrate that it is crucial to consider the interaction between ion fluxes as treating the electrolytes independently, as is sometimes proposed, does not completely capture the dynamics of ion transport. We validate our numerical predictions by conducting experiments with sodium fluorescein salt in the pore and sodium chloride/sodium sulphate/sodium hydroxide in the reservoir. Our visualization and results demonstrate that ion diffusivities and concentrations in the reservoir can influence the diffusion rates of fluorescein, which underscores that ion fluxes are coupled and that multiple electrolytes cannot be treated independently. These results should be useful to the wide range of situations where concentration variations are imposed on systems with an existing background electrolyte.

Diffusiophoretic Particle Penetration into Bacterial Biofilms.

Bacterial biofilms are communities of cells adhered to surfaces. These communities represent a predominant form of bacterial life on Earth. A defining feature of a biofilm is the three-dimensional extracellular polymer matrix that protects resident cells by acting as a mechanical barrier to the penetration of chemicals, such as antimicrobials. Beyond being recalcitrant to antibiotic treatment, biofilms are notoriously difficult to remove from surfaces. A promising, but relatively underexplored, approach to biofilm control is to disrupt the extracellular polymer matrix by enabling penetration of particles to increase the susceptibility of biofilms to antimicrobials. In this work, we investigate externally imposed chemical gradients as a mechanism to transport polystyrene particles into bacterial biofilms. We show that preconditioning the biofilm with a prewash step using deionized (DI) water is essential for altering the biofilm so it takes up the micro- and nanoparticles by the application of a further chemical gradient created by an electrolyte. Using different particles and chemicals, we document the transport behavior that leads to particle motion into the biofilm and its further reversal out of the biofilm. Our results demonstrate the importance of chemical gradients in disrupting the biofilm matrix and regulating particle transport in crowded macromolecular environments, and suggest potential applications of particle transport and delivery in other physiological systems.

CO2-Driven diffusiophoresis and water cleaning: similarity solutions for predicting the exclusion zone in a channel flow.

We investigate experimentally and theoretically diffusiophoretic separation of negatively charged particles in a rectangular channel flow, driven by CO2 dissolution from one side-wall. Since the negatively charged particles create an exclusion zone near the boundary where CO2 is introduced, we model the problem by applying a shear flow approximation in a two-dimensional configuration. From the form of the equations we define a similarity variable to transform the reaction-diffusion equations for CO2 and ions and the advection-diffusion equation for the particle distribution to ordinary differential equations. The definition of the similarity variable suggests a characteristic length scale for the particle exclusion zone. We consider height-averaged flow behaviors in rectangular channels to rationalize and connect our experimental observations with the model, by calculating the wall shear rate as functions of channel dimensions. Our observations and the theoretical model provide the design parameters such as flow speed, channel dimensions and CO2 pressure for the in-flow water cleaning systems.

Dissolution without disappearing: multicomponent gas exchange for CO2 bubbles in a microfluidic channel.

We studied the dissolution dynamics of CO2 gas bubbles in a microfluidic channel, both experimentally and theoretically. In the experiments, spherical CO2 bubbles in a flow of a solution of sodium dodecyl sulfate (SDS) first shrink rapidly before attaining an equilibrium size. In the rapid dissolution regime, the time to obtain a new equilibrium is 30 ms regardless of SDS concentration, and the equilibrium radius achieved varies with the SDS concentration. To explain the lack of complete dissolution, we interpret the results by considering the effects of other gases (O2, N2) that are already dissolved in the aqueous phase, and we develop a multicomponent dissolution model that includes the effect of surface tension and the liquid pressure drop along the channel. Solutions of the model for a stationary gas bubble show good agreement with the experimental results, which lead to our conclusion that the equilibrium regime is obtained by gas exchange between the bubbles and liquid phase. Also, our observations from experiments and model calculations suggest that SDS molecules on the gas-liquid interface form a diffusion barrier, which controls the dissolution behaviour and the eventual equilibrium radius of the bubble.