4.2. Temperature gradient

liquids ([BMIM][PF6] and [BMIM][Tf2N]) as heating elements. The ionic liquid was Joule heated by an AC current (power: 1 W, frequency: 50 Hz, and voltage: ≤ 3.75 kV). By controlling the applied voltage of the ionic liquids, the bulk temperature was regulated from 50 to 90C

Figure 6. Top view of the microchip showing the patterned heaters and temperature sensors [63].

Other than the commonly used physical heating approach (i.e., Joule heating), chemical approach can be another integrated option. To locally control the temperature in a microchannel, Guijt et al. [66] made use of heat adsorption and dissipation via endothermic and exothermic chemical reactions, respectively (Figure 7a). Heating/cooling can be achieved in a temperature control channel (TCC) where two reagents from separate reactant channels (RC1 and RC2) merged and had chemical reaction. Heating was achieved with a dissolution of 97 wt% H2SO4 (Reagent 1) in water (Reagent 2), while cooling was realised with the evaporation process acetone (Reagent 1) in the air (Reagent 2). A wide temperature range from 3 to 76C with a ramp rate of 1C/s can be realised in the central channel by manipulating the flow rate ratio

Figure 7. (a) Two reactant channels (RC) merging into a temperature control channel (TCC) and (b) layout of the device

with an accuracy of 0.2C.

114 Microfluidics and Nanofluidics

between two reagents (shown in Figure 7b).

used for demonstration experiments [66].

For several applications such as Soret effect and droplet actuation, temperature gradients are demanded in microfluidic systems. Temperature gradients have been generated in either a controlled temperature profile or an arbitrary way by various techniques.

As illustrated in Figure 8a, Mao et al. [67] generated a linear temperature gradient along the horizontal direction of the PDMS microchannel by using the pre-heated liquid approach. Hot and cold fluids were channelled into two side channels separately, and a stable temperature gradient has been established in the central channel (Figure 8b). Zhao et al. [68] implemented this approach for studies on thermophoresis using a microchip made of stainless steel. Due to a higher conductivity of stainless steel than PDMS, a wider range of variations for both bulk temperature and temperature gradient (1.5 104 K/m) can be formed in the microchannel. In order to fabricate similar structures in PDMS microchip, Yan et al. [69] proposed a fast prototyping method for single-layer PDMS microfluidic devices with abrupt depth variations by combining the laser ablation and NOA81 moulding. The whole fabrication process can be completed within 2 h. This method can readily produce PDMS microfluidic devices with micrometre and millimetre structures in one step. Moreover, this method can be applied in a non-clean-room environment and does not require complicated and expensive soft lithography equipment or etching processes.

Instead of preheating liquids, Vigolo et al. [70] chose to use silver-filled epoxy with a similar parallel channel configuration to generate temperature gradients in a microchannel. This microscale heating element can be powered by two ordinary AAA batteries. Thus, this design shows its potential as a cost effective and portable solution for thermal control of microfluidic devices. Alternatively, the parallel-channel configuration has also been implemented for control of bulk temperatures in microchannels. It can be readily realised by changing the directions of preheated liquid and DC electric current [71–73].

In addition to regulating bulk temperatures, Peltier elements can be utilised to establish a constant temperature gradient in microfluidic systems. Matsui et al. [74] designed and fabricated a hybrid temperature gradient focusing (TGF) chip (materials: PDMS/glass) by applying two Peltier elements. A range of temperature gradient can be formed along the horizontal

Figure 8. (a) Schematic diagram of a device with an on-chip linear temperature gradient and (b) a plot of temperature vs. position of the temperature gradient device [67].

direction of the microchannel (Figure 9). The maximum temperature gradient generated in their device was 13.75C/mm.

Jiao et al. [75] designed a microfluidic droplet manipulation system with integration of four microheaters (materials: titanium and platinum) generating planar temperature gradients in the square region (10 mm 10 mm). By controlling the frequency and amplitude of square wave signals, they successfully manipulated the trajectory of a microdroplet based on the periodic thermocapillary actuation caused by temperature gradients. Yap et al. [76] deposited a thin film platinum heater in a microchannel to control the thermal field for droplet formation in a bifurcated microchannel. The trajectory and splitting of the droplet can be controlled by modulating viscosity and interfacial tension of liquids under different heating conditions. Because of the temperature gradients generated by metal microheaters, the PDMS microchannel would undergo a slight dilation. The temperature-induced dilation changed the height of the microchannel so that the bubble would be driven away from the constricted region. Selva et al. [77] successfully controlled the bubble motion by utilising this thermomechanical actuation under temperature variation. Miralles et al. [78] from the same research group optimised their previous design by using localised heating resistor whose size was smaller than the droplet size.

The aforementioned designs of temperature-gradient devices are not suitable for researches on the microscale mechanism of particle deposition in microchannels due to the obstruction of heating or cooling elements along the optical path of microscope for observation or inconsistency of the directions of temperature gradient and particle deposition. The authors' group has developed a novel design of temperature-gradient microchip to investigate particle deposition in microchannels [79].

The microchannel can be fabricated with polydimethylsiloxane (PDMS) using standard soft lithography techniques. The PDMS monomer and the curing agent are fully mixed in a mass ratio of 10:1, and then they are vacuumed for 45 min to evacuate air bubbles remaining in the PDMS mixture. The mixture is applied onto the master mould constructed by SU8 on a silicon wafer. PDMS is cured after being heated with the wafer in an oven at 80C for about 1 h. A thin layer of polymerised PDMS (e.g., 1 mm in thickness) is peeled off from the mould. Two cylindrical openings are then punched at both ends of the microchannel as the inlet and outlet

Figure 10. Schematic of the temperature-gradient microchip, consisting of the PDMS microchannel, the glass slide coated with the ITO film and the TEC unit. Various temperature gradients can be achieved by cooling the top surface of the microchip with the TEC unit and heating the bottom surface with the ITO film heater. (figure is for not drawn to

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117

For the ITO glass slide, a thin indium tin oxide film (e.g., 200 nm in thickness) is deposited on one side of a glass slide (e.g., 0.71 mm in thickness) as a heater for the microchip. Indium tin oxide is a solid mixture including 90% In2O3 and 10% SnO2 by mass. On the other side of the glass slide, a thin PDMS film was coated on the bare glass surface. The ITO glass heater has three major advantages for the particle deposition study. (1) The ITO glass (10 ohms/sq) can easily generate heat by Joule heating when being connected into an electrical circuit. The heat dissipation rate of the ITO glass can be well controlled by regulating the applied electrical current and voltage. (2) The ITO glass has excellent optical transparency allowing direct observation on particle deposition onto the bottom surface of the microchannel along the direction of the applied temperature gradient. The bottom view of the particle deposition can be readily captured via using an inverted microscope equipped with a CCD camera. (3) A wide range of customised dimensions and patterns can be precisely achieved for ITO heaters

A closed microchannel is formed via irreversibly oxygen plasma bonding the treated ITO glass slide and the PDMS block with microchannel structure. The bonded microchannel is heated in

by implementing the standard photolithography techniques.

for sample fluids.

scale) [79].

There are three major parts for the temperature-gradient microchip (Figure 10), including a microchannel, a thermoelectric cooler (TEC) unit as cold end, and a thin glass slide coated with indium tin oxide (ITO) film as hot end. A temperature gradient can be established along the vertical direction inside the microchannel, by cooling the top surface of the microchip with the TEC unit and heating the bottom surface with the ITO film heater.

Figure 9. Schematic drawing of temperature gradient focusing apparatus [74].

direction of the microchannel (Figure 9). The maximum temperature gradient generated in

Jiao et al. [75] designed a microfluidic droplet manipulation system with integration of four microheaters (materials: titanium and platinum) generating planar temperature gradients in the square region (10 mm 10 mm). By controlling the frequency and amplitude of square wave signals, they successfully manipulated the trajectory of a microdroplet based on the periodic thermocapillary actuation caused by temperature gradients. Yap et al. [76] deposited a thin film platinum heater in a microchannel to control the thermal field for droplet formation in a bifurcated microchannel. The trajectory and splitting of the droplet can be controlled by modulating viscosity and interfacial tension of liquids under different heating conditions. Because of the temperature gradients generated by metal microheaters, the PDMS microchannel would undergo a slight dilation. The temperature-induced dilation changed the height of the microchannel so that the bubble would be driven away from the constricted region. Selva et al. [77] successfully controlled the bubble motion by utilising this thermomechanical actuation under temperature variation. Miralles et al. [78] from the same research group optimised their previous design by using localised heating resistor whose size was smaller

The aforementioned designs of temperature-gradient devices are not suitable for researches on the microscale mechanism of particle deposition in microchannels due to the obstruction of heating or cooling elements along the optical path of microscope for observation or inconsistency of the directions of temperature gradient and particle deposition. The authors' group has developed a novel design of temperature-gradient microchip to investigate particle deposition

There are three major parts for the temperature-gradient microchip (Figure 10), including a microchannel, a thermoelectric cooler (TEC) unit as cold end, and a thin glass slide coated with indium tin oxide (ITO) film as hot end. A temperature gradient can be established along the vertical direction inside the microchannel, by cooling the top surface of the microchip with the

TEC unit and heating the bottom surface with the ITO film heater.

Figure 9. Schematic drawing of temperature gradient focusing apparatus [74].

their device was 13.75C/mm.

116 Microfluidics and Nanofluidics

than the droplet size.

in microchannels [79].

Figure 10. Schematic of the temperature-gradient microchip, consisting of the PDMS microchannel, the glass slide coated with the ITO film and the TEC unit. Various temperature gradients can be achieved by cooling the top surface of the microchip with the TEC unit and heating the bottom surface with the ITO film heater. (figure is for not drawn to scale) [79].

The microchannel can be fabricated with polydimethylsiloxane (PDMS) using standard soft lithography techniques. The PDMS monomer and the curing agent are fully mixed in a mass ratio of 10:1, and then they are vacuumed for 45 min to evacuate air bubbles remaining in the PDMS mixture. The mixture is applied onto the master mould constructed by SU8 on a silicon wafer. PDMS is cured after being heated with the wafer in an oven at 80C for about 1 h. A thin layer of polymerised PDMS (e.g., 1 mm in thickness) is peeled off from the mould. Two cylindrical openings are then punched at both ends of the microchannel as the inlet and outlet for sample fluids.

For the ITO glass slide, a thin indium tin oxide film (e.g., 200 nm in thickness) is deposited on one side of a glass slide (e.g., 0.71 mm in thickness) as a heater for the microchip. Indium tin oxide is a solid mixture including 90% In2O3 and 10% SnO2 by mass. On the other side of the glass slide, a thin PDMS film was coated on the bare glass surface. The ITO glass heater has three major advantages for the particle deposition study. (1) The ITO glass (10 ohms/sq) can easily generate heat by Joule heating when being connected into an electrical circuit. The heat dissipation rate of the ITO glass can be well controlled by regulating the applied electrical current and voltage. (2) The ITO glass has excellent optical transparency allowing direct observation on particle deposition onto the bottom surface of the microchannel along the direction of the applied temperature gradient. The bottom view of the particle deposition can be readily captured via using an inverted microscope equipped with a CCD camera. (3) A wide range of customised dimensions and patterns can be precisely achieved for ITO heaters by implementing the standard photolithography techniques.

A closed microchannel is formed via irreversibly oxygen plasma bonding the treated ITO glass slide and the PDMS block with microchannel structure. The bonded microchannel is heated in 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 deposition along the direction of applied temperature gradient.

resulted from the neutralisation effect of the electrolyte concentration (10<sup>6</sup>

of the cationic surfactant concentration (CTAB, 10<sup>6</sup>

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

performance in the glass microchannel.

be the most plausible reason for such discrepancies.

system.

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

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

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

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

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

–10<sup>3</sup> M) which

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–10<sup>4</sup> M) but was reduced monotonically

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