Why do liquid crystal plastic films show various colours after contacting hot substances?
To understand the phenomenon, we have to note one thing first: “Only the light waves with a pitch of an integral multiple of the wavelength could be reflected by liquid crystals.” [1] When we touch the plastic film, heat is passed to the liquid crystal, causing its pitch to shorten. When the pitch reaches a multiple of the wavelength of a certain colour light, we could see the light of that particular colour at the place while visible lights of other colours are absorbed (Fig. 2). Thus liquid crystals may indicate its surrounding temperature by means of colour. When different types of liquid crystals are combined into use, it could act as a sensor that detects various temperature differences, which could be used in checking cracks in an electric circuit board, the pattern of fluid motion, the state of a battery, the strength of radiation, or even the “mood-detecting rings” depicted in fictions.
