5.1.3. Effect of properties of particle and wall on particle deposition

the oven to reinforce the plasma-bonding strength. The thermoelectric cooler (TEC) unit is mounted on the top of the bonded microchannel to provide a stable cold end over the microchannel. The temperatures of hot/cold ends can be readily adjusted via the DC power supplies for the ITO film heater and the TEC unit, respectively. Moreover, the temperature gradient can be well controlled in the microchannel along the same direction as the particle deposition. Thermal conductive silicon paste can be applied in the gap between the TEC unit and the PDMS microchip to enhance the heat conduction. This specially designed temperaturegradient microfluidic system provides a useful tool for researches on dynamics of particle deposition under different thermal conditions. To the best knowledge of the authors, it could be the first microfluidic device allowing to directly observe the dynamic process of particle deposi-

Particle deposition onto solid surfaces has been intensively studied in both experimental and theoretical approaches. In this section, major works on particle deposition in microchannels are

Experiments are usually conducted in three types of setup, including cylindrical, parallel-plate channels, and impingement jet chamber. Various parameters have been evaluated experimentally in terms of their effects on particle deposition, such as pH, electrolyte concentration,

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.

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)

tion along the direction of applied temperature gradient.

5. Particle deposition in microchannels

reviewed.

118 Microfluidics and Nanofluidics

5.1. Experimental studies

particle, and solid surface.

5.1.1. Effect of pH on particle deposition

5.1.2. Effect of the ion concentration on particle deposition

Salim et al. [85] investigated the effects of protein (fibrinogen and lysozyme) adsorption on the electroosmotic flow (EOF) behaviour of the plasma-polymerised glass microchannel surfaces. Three types of plasma-polymerised surfaces (pp.TG, pp.AAm, and pp.AAc) were tested which had different surface charges and charge densities. They observed a non-fouling phenomenon with tetraglyme coating in the presence of protein, and this coating provided stable EOF performance in the glass microchannel.

Mustin and Stoeber [86] conducted experiments with polystyrene microsphere suspensions in a PDMS microchannel. They found that the dynamics of channel blockage was influenced by particle size distribution besides the particle size alone. Recently, they performed another experiment in a mini impingement jet flow cell made of PDMS for particle deposition (Figure 11) [87]. They noticed discrepancies between the experimental measurements and numerical simulation results based on both DLVO and extended DLVO theories, and proposed the surface roughness and electrostatic charge heterogeneity of the PDMS surface could be the most plausible reason for such discrepancies.

Figure 11. (a) Cross-section view of the deposition chamber and (b) flow cell on substrate without clamping [87].
