Liquid-core/liquid-cladding Optical Waveguides Using Thermal Gradients Formation and Their Application to Chemical Analysis

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Proposed was a liquid-core/liquid-cladding optical waveguide using thermal gradients (TG-LLW), where laminar flow of water in a stainless capillary tube was placed in a heat source, and thermal gradients across the laminar flow were formed. Laminar flow of water was formed in a stainless steel tube (i.d., 0.8 mm; o.d., 1.6 mm, 15 cm long) using an HPLC pump, whose flow rate was 0 to 9 cm^3 min^<-1> (the average linear velocity of the flow was 0 to 30 cm s^<-1>). The stainless tube was placed in a heat source, i.e., circulated hot water, whose temperature was changed from room temperature to 70 degree. In the experiments, the higher the temperature of the heat source was and the greater the flow rate, the higher intensity of the guided light was obtained. CFD simulation showed that the increase in the temperature of the heat source resulted in the increase in thermal gradients in the flow, i.e., the increase in refractive index gradients in the flow. Moreover, the CFD calculation also elucidated the effect of the flow rate. The guided light intensity changed periodically with the distance, and the period became shorter with the increase in the temperature of the heat source. Thisphenomenon is explained by the theory of graded index optical fiber. That is, the smaller refractive index gradient results in the longer period of the guided light. This nature of TG-LLW should be useful to manipulate the light, i.e., focusing and dispersing guided light in optofluidic systems in future.