Compact MEMS-based sensor for refractive index monitoring of liquids

This work introduces and verifies a novel sensor design for measuring the refractive index of liquids. Due to the compact size and efficient fabrication, this type of sensor is specifically suited, e. g. as part of industrial condition monitoring, for the continuous determination of liquid mixture concentrations. Utilizing micro-electromechanical systems (MEMS) technology, the sensor combines cost-effective manufacturing, robustness, and versatility in liquid characterization. The measurement is based on the different degrees of reflection of a collimated light beam at different positions along a curved interface between the silicon platform and the liquid. As a result, the beam undergoes an overall approximately linear intensity loss, which is highly sensitive to the refractive index of the medium to be analyzed. The measuring range and sensitivity of the sensor can be adjusted to accommodate various applications. This paper presents detailed design principles, outlines the manufacturing process, and supports analytical considerations with rigorous numerical simulations. Experimental studies conducted using various mixtures of water with solvents such as Ethylene Glycol and Glycerol (commonly used as an industrial coolants) confirm the viability of the sensor system for real-world applications, highlighting its potential as a reliable tool for condition monitoring in liquid-based systems. • Design, simulation, and fabrication of a refractive-index sensor for liquids. • Measurement using partial total internal reflection on curved surfaces in near IR. • Cost-effective production using silicon and standard MEMS processing techniques. • Innovative design offers an adjustable measurement range with almost linear signal.