5.1.1. Effect of pH on particle deposition

The repulsive interaction would exist between charged particles of same like sign and hinder fouling behaviour. The zeta potentials of the particle and microchannel surface determine the magnitude of the repulsion and are closely related to the pH value of working fluid. The pH value can control the charge signs of the particle and channel surface. Newson et al. [80] investigated the mechanisms of deposition, removal and sticking in a haematite/water system (particle diameter: 0.2 μm and concentration: 100 ppm). They found that the sticking coefficient of particles from turbulent water suspension (Re: 11,000–14,000) was significantly dependent upon the pH value of suspension. Perry and Kandlikar [36] also adjusted the pH value of the nanofluids and were able to effectively mitigate particulate fouling. The reduction of the deposition rate was mainly attributed to the augmented EDL repulsion as pH was increased.

### 5.1.2. Effect of the ion concentration on particle deposition

Gu and Li studied the influence of the electrolyte concentration on deposition of silicon oil microdrops onto cylindrical surfaces via both experimental and numerical approaches [81, 82]. They found that the increase of the Sherwood number (dimensionless mass transport rate) resulted from the neutralisation effect of the electrolyte concentration (10<sup>6</sup> –10<sup>3</sup> M) which crucially determined the zeta potentials of bare glass tube surfaces and silicon oil drops. Moreover, they found that the Sherwood number was increased significantly with the increase of the cationic surfactant concentration (CTAB, 10<sup>6</sup> –10<sup>4</sup> M) but was reduced monotonically with adding anionic surfactant (SDS) into the oil droplet emulsion. Kar et al. [83] performed experiments with CaCO3 microparticles and hollow fibre membrane to study the effect of salt concentration gradients on particle deposition. They reported that the diffusiophoretic particle transport has crucial influence on particle deposition when different electrolyte ions of salts in solution have different diffusion coefficients. Furthermore, Guha et al. [84] found the diffusiophoresis has significant influence on the colloidal fouling in a low salinity reverse osmosis system.
