Thermal modulators, phase-coated

FIGURE 5.9 Phase-coated thermal modulators using a rotating heater (a), moving cold zone (b), and cold gas jets (c) to modulate the concentration of the sample vapor entering the device. For configuration (c), the jets are turned on and off alternately to modulate the sample vapor concentration. 

A variation on the method of thermal modulation is the use of a length of capillary column coated with a thick film of stationary phase. At ambient oven temperatures, this results in a retention of semivolatile analytes, which may be subsequently released to the secondary column once the trap is heated. The rapid cycling time possible with this methodology has resulted in its common application as the intermediate trap in comprehensive GC. 

Based on the definition of the capillary number, the two-phase flow patterns in microchannels can be influenced by interfacial forces (fluid viscosity (rf), flow velocity (v), and geometric features (G). The interfacial forces can be modulated by adding surfactants, surface coating, and patterning techniques but also by electrical, thermal, or optical modulations of the interfacial... 

A typical thermally induced light valve consists (Fig. 49a) of a thin layer (14 [im) of cholesteric liquid crystal sandwiched between two substrates coated with visibly transparent IR-absorbing electrodes such as In2 xSnx03 y. An X—Y deflected intensity-modulated focused IR laser beam is absorbed by the electrodes, thereby locally heating the liquid crystal from its well-ordered transparent planar cholesteric state to the transparent isotropic phase as shown in Fig. 49b. Cooling occurs almost... 

Another method is photothermal excitation. In the method, a cantilever is coated with an Au thin film. A laser beam with a power modulated at /o is irradiated to the cantilever. The modulated laser beam causes thermal expansion of the Si (or SisN4) cantilever and the Au film. Owing to the difference in the thermal expansion coefficient between the two materials, the laser beam generates a stress at the cantilever-film interface, which induces the cantilever vibration S3mchronized with the laser power modulation. In this case, the cantilever is directly driven by the laser beam so that the amplitude and phase curves are free from the influence of the spurious resonances. 

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