Experimental study on thermophoresis of colloids in aqueous surfactant solutions

Abstract

Thermophoresis refers to the motion of particles under a temperature gradient and it is one of the particle manipulation techniques. Regarding the thermophoresis of particles in liquid media, however, many open questions still remain, especially the role of the interfacial effect. This work reports on a systematic experimental investigation of surfactant effects, especially the induced interfacial effect, on the thermophoresis of colloids in aqueous solutions via a microfluidic approach. Two kinds of commonly used surfactants, sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB), are selected and the results show that from relatively large concentrations, the two surfactants can greatly enhance the thermophilic mobilities. Specifically, it is found that the colloid-water interfaces modified with more polar end groups can potentially lead to a stronger thermophilic tendency. Due to the complex effects of surfactants, further theoretical model development is needed to quantitatively describe the dependence of thermophoresis on the interface characteristics.

Thermophoretic Micron-Scale Devices: Practical Approach and Review

“…Alternatively, heating and cooling channels can be used with a counter flow (cf. Figure 1 c), so that the temperature gradient remains constant along the measurement channel, but a change of the average temperature in z -direction cannot usually be avoided [ 43 , 45 , 59 ]. An example of a channel geometry will be given in Section 5.2.1 .…”

“…If the dimensions and thermophysical properties are known the temperature profile between the cold and hot channel can be estimated. The micron-scale devices using channels do not require clean-room facilities, as it is required for lithographical fabrication on nano-size wires and has been realized in various labs [ 41 , 43 , 45 , 47 , 55 , 56 , 58 , 59 , 61 ].…”

“…In the majority of the devices [ 41 , 43 , 45 , 47 , 55 , 56 , 58 , 59 ], water is directly used as thermostating fluid or as heat sink fluid, if Peltier elements are used [ 61 ]. So far, the operating temperatures are located in the liquid state of water.…”

“…A material with a high thermal conductivity ( W/mK) should be preferred in order to reduce the temperature change across the gap. For this reason, materials such as copper ( W/mK), sapphire ( W/mK) [ 41 , 47 , 55 , 56 ], aluminum ( W/mK) [ 58 ] and stainless steel ( W/mK) [ 43 , 45 , 59 ] are used. However, except for sapphire, most materials with a high thermal conductivity are not transparent for visible light and, therefore, they are not suitable for optical detection.…”

“…There have been several approaches to miniaturize thermophoretic cells [ 27 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 ]. Many instruments using laser light are operated in combination with microscopes, while others use the deflection of laser beams or interference due to refractive index contrast [ 38 , 41 , 47 ].…”