Accumulation of Colloidal Particles 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.

Popular Summary

Underground aquifers, hydraulic fractures, coastal habitats, and water filtration systems all deal with the flow of water that contains suspended particles. Typically, these particles are expected to move along with the fluid flow unless the particles are large enough to clog the channel or sticky enough to adhere to boundaries. However, we observe that this is not always the case when there are solutes dissolved in the fluid. Our experiments show that the particles can get trapped and can accumulate rapidly in the flow junctions because of a combination of solute gradients and fluid flow.

When two flows with different solute concentrations are in contact near a junction, a sharp solute gradient is formed at the interface, allowing the particles to move against the flow direction. Consequently, the particles accumulate near the pore entrance, rapidly occupying the pore. 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.

Possible practical examples of this particle trapping include developing microbial biofilms and biofilm streamers in certain natural habitats, as well as interrupting oral and gastrointestinal microbial infections in the human body. We also show that this effect can be exploited as a useful tool for preconcentrating biomolecules for rapid bioassays.

Figure 1

Merging of two flow streams with different solute concentrations can lead to rapid accumulation of colloidal particles. (a) Particle accumulation near a flow junction due to the balance between diffusiophoretic motion (DP) and the fluid flow. (b) Schematic of the microfluidic channel for testing flow-induced diffusiophoretic particle accumulation. Pressure $p$ in the upper channel (solution 1) is larger than in the lower channel (solution 2), so solution 1 permeates through the pore and merges with solution 2. (c) Fluorescence image sequence of the particles [polystyrene (PS), $\text{diameter}=0.5\mu \mathrm{m}$] accumulating near the bottom flow junction (Movie 1 in Ref. [6]). The solute (NaCl) concentration in the upper stream is $c_1=10\mathrm{mM}$, whereas the lower stream has $c_2=0.1\mathrm{mM}$. The length of the pore is $L=400\mu \mathrm{m}$. (d) Spatiotemporal plot of the width-averaged fluorescence intensity within the pore. (e) Evolution of the local maximum fluorescence intensity normalized by the intensity at $t=0\mathrm{s}$ ($I/I_0$). (f) Bright-field imaging of the particle accumulation near the flow junction (Movie 2 in Ref. [6]). Densely packed colloidal clusters are clearly seen after 1 minute of the experiment. All scale bars are $50\mu \mathrm{m}$.

Figure 2

Flow-induced diffusiophoretic accumulation occurs regardless of the particle size or the flow speed. Image sequences of (a) polystyrene nanoparticles (NP, $\text{diameter}=50\mathrm{nm}$) focusing near the flow junction and (b) polystyrene particles ($\text{diameter}=0.5\mu \mathrm{m}$) focusing near the flow junction at a higher pore flow speed $u_f$. In both cases, the solute concentration in the upper stream is $c_1=10\mathrm{mM}$, and the solute concentration in the lower stream is $c_2=0.1\mathrm{mM}$. The scale bar is $50\mu \mathrm{m}$.

Figure 3

3D numerical simulations of the diffusiophoretic particle dynamics. (a) Particle trajectories on a slice taken through the center of the pore. Particles approach a position within the pore where fluid velocities are balanced by diffusiophoretic velocities. Ultimately, particles travel towards two accumulation points near the sides of the pore. (b) Computed particle concentrations normalized by the initial concentration ($n/n_0$) over time, showing accumulation near the stable points seen in panel (a). Large white arrows indicate the mean flow direction. The scale bar is $50\mu \mathrm{m}$.

Figure 4

Small particles can lead to irreversible clogging in large pores. Image sequence of small particles ($0.5\mu \mathrm{m}$ in diameter) accumulating near the large pore mouths ($50\mu \mathrm{m}$ in width, see Movie 3 in Ref. [6]) due to the outflow of the upper concentrated stream. The solute concentration of the upper stream is $c_1=100\mathrm{mM}$, and the solute concentration of the lower stream is $c_2=0\mathrm{mM}$. Only the lower stream contains polystyrene particles. The bottom-right panel shows clogged particles near the pore mouths after the flow direction within the pores has been reversed. The inset shows a close-up image of the black dashed box. The scale bar is $100\mu \mathrm{m}$.

Figure 5

Automated sample preconcentrator for a rapid bioassay. (a–c) A conceptual schematic of the preconcentrator using the flow-induced diffusiophoretic accumulation. An array of pores operates as a trap for the analytes. (a) Upon initiation of the outflow through the pores due to the higher pressure $p_1$ at the upper channel than at the lower channel ($p_2$), the dilute analytes are rapidly concentrated near the flow junctions, and a stable detection can be made in this region. (b) The accumulated analytes can be transferred to the other side of the channel by changing the pressure difference across the pores for further mixing with some other chemicals or for flow-based detection. (c) Subsequently, the analytes can be analyzed at the downstream. (d–h) Experimental results showing fluorescence image sequence of preconcentration and transfer of $\lambda$-DNAs using the flow-induced diffusiophoretic accumulation (Movie 4 in Ref. [6]). Each image corresponds to the time indicated in panel (i). The solute concentration of the upper stream is $c_1=10\mathrm{mM}$, and the solute concentration of the lower stream is $c_2=0.1\mathrm{mM}$. Both streams contain $\lambda$-DNAs. (i) Fluorescence intensity measurement near the flow junction (orange curve) and downstream in the upper channel (blue curve). The scale bar in panel (h) is $100\mu \mathrm{m}$.