Surface Tension and Concentration Measurement of Sub-${\mu}$L Solution Using a Cantilever-Based Optical Gauging System

A cantilever sensor integrated with an on-tip micro spherical reflecting mirror (MSRM) exhibits a larger optical beam displacement than a conventional one, i.e., the system sensitivity/responsivity is enhanced. In this paper, it is employed as a surface tension and concentration gauge that only requires 0.5 μL of solution. The MSRM-integrated cantilever is first set above a sodium chloride-water droplet carried by a glass substrate, and then the droplet is moved up gradually. Once the cantilever is touched by the droplet, it is pulled and bent down as the droplet reshapes. The cantilever deformation amount is related to the surface tension of the solution, which increases with the molar concentration of sodium chloride. According to our experiments, the surface tension varies from 72.1 to 77.7 mN/m as the molar concentration of sodium chloride in water increases from 0 to 3.13 M. Therefore, by measuring the bending amount of the cantilever, the surface tension as well as the concentration of the NaCl-water solution can be determined. We also perform the experiments on the alcohol (ethanol)-water mixture, whose surface tension, conversely, reduces from 71.4 to 57.5 mN/m as the alcohol molar concentration increases from 0 to 0.81 M.