Frequency response of a surface thermometer based on unencapsulated thermochromic liquid crystals

Thermochromic liquid crystals (TLCs) are used by many researchers for temperature field analysis in fluid dynamic experiments. This study is targed on the transient behaviour of a TLC system. Therefore, unencapsulated TLCs are filled into flat microchannels by capillary forces, whereby different thicknesses of the TLC layer could be realised. On the back side of the TLC channel a thin metallic foil is used as a plane electric heater, which allows harmonic stimulation at variable frequencies. Evaluation of the TLC temperature response was realised by analysing the fluctuation of the TLC colour play using a high speed colour camera with a frame rate of 1000 Hz. In a test series different TLC layers (10, 20 and 30 μm thick) were investigated with harmonic stimulations at frequencies between 3 and 70 Hz. Additionally, the specific heat of the pure TLC substance was measured accurately with a differential scanning calorimeter (DSC). The system was described with an analytical approach and modelled with a MATLAB partial differential equation (PDE) tool.It was figured out that inside the analysed frequency band the amplitude of the TLC temperature response is attenuated and the phase is shifted in contrast to the temperature of the plain heating foil. The ratio of the amplitudes is about 20% with a 30 μm thick TLC layer and about 35% with a 10 μm TLC layer. The maximum frequency where a detectable sine curve could be displayed was around 70 Hz. Validation with numerical results shows that most of the amplitude attenuation and phase shift is caused by heat conduction. A residual permanent phase shift of about 1 ms might be caused by an averaging process over the thickness of the TLC layer, or it is an intrinsic physical lag of the TLC molecules. However, for transient processes inside the analysed frequency range numeric calculations or solutions of the inverse heat conduction problem are required to obtain the actual temperature of the heater surface.