**Introduction:** We experimentally study the effects of salt concentration on the flowing dynamics of dense suspensions of micrometer-sized silica particles in microfluidic drums. In pure water, the particles are fully sedimented under their own weight, but do not touch each other due to their negative surface charges. Adding ions in solution screens the repulsive forces between particles, and the flowing properties of the suspension are heavily modified. We interpret the results in terms of an effective increase of the inter-particles friction and of the pile compacity at rest.

## Experimental set-up

We use a custom-build vertical microscope, to visualize microfluid drums (100 µm diameter) filled with 2.12 µm colloidal silica particles.

<figure><img src="image_1.png" /></figure>

We start from sedimented piles of particles. At at time $t = 0$ we suddenly rotate the drums, and track the time evloution of the pile angle $\theta$ with respect to the horizontal.

Our samples allow us to track up to ~20 drums simultaneously.

## Main results

<figure><img src="image_2.png" /></figure>

We observe that the increase in salinity induce the three following effects:

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

## Compacity

The delay in starting time $\tau_s$, can be explained by a dilatancy effect due to an increase of the initial compacity of the pile[1]. To test this hypothesis we have measured the compacity of piles at rest with confocal microscopy:

<figure><img src="image_6.png" /></figure>

The typical volume occupied by a particle gives access to the compacity at rest $\phi_0$:

* in pure water $\phi_0 = 51\%$,

* in $10^{-2}$ mol/L saline solution $\phi_0 = 61\%$.

## Friction

The repulsive layer around each particle has a typical size of the Debye length $\lambda_D$. Following the frictional transition scenario[2], when this length becomes of the same size as the typical surface roughness $r$, the particles become more frictionnal. An increase of the microscopic friction coefficient between the particles $\mu_P$ is known to increase both the repose angle of a pile, and the viscosity of the suspension.

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

Thanks to AFM measurements, we have verified that $\lambda_D \approx r$ when the ionic strength is about $2.3 \times 10^{-2}$ mol/L.

## Take home message

Flowing properties of dense colloidal suspensions are modified when inter-particles repulsion is screened by salt addition. When increasing the ionic strength of the solution, we observe a continuous increase of:

* the initial delay $\tau_s$ before the pile starts to flow

* the critical angle $\theta_c$ between the two flowing regimes ("avalanche" and "creep")

* the time $\Delta t$ required for the pile to flow of an angle $\Delta\theta$ in the avalanche regime

All those observations are consistent with a continuous increase of the initial packing fraction of the pile $\phi_0$, and of the microscopic friction coefficient $\mu_P$ between the particles.

Results published in:
*Soft Matter*, 2024, **20**, 3367-3375,
DOI: 10.1039/D4SM00035H

<figure><img src="image_9.png" /></figure>

This work has been funded by the French Agence Nationale de la Recherche (ANR), under grant ANR-21-CE30-0005 (JCJC MicroGraM).

<footnote>[1] M. Pailha, M. Nicolas and O. Pouliquen, *Phys. Fluids*, 2008, **20**, 111701</footnote>

<footnote>[2] H. Perrin, C. Clavaud, M. Wyart, B. Metzger, and Y. Forterre *Phys. Rev. X* **9**, 031027 (2019).</footnote>