Viscosity measurement based on the tapping-induced free vibration of sessile droplets using MEMS-based piezoresistive cantilevers.

We report a simple technique to measure the free vibration of microlitre-sized droplets using an array of thirteen MEMS-based piezoresistive cantilevers and demonstrate its application for the measurement of viscosity. Because the damping of the free vibration of a liquid droplet is known to be affected by the viscosity of the liquid, measuring the vibration of a droplet allows the viscosity to be estimated from a dilute sample volume. However, conventional methods to measure the droplet vibration require sophisticated apparatuses, which hinder the development of a portable viscometer. Here, we show that MEMS-based piezoresistive cantilevers can be an excellent tool to measure the vibration of a sessile droplet due to the high sensitivity and simplicity of the readout scheme. Using the cantilever array, we analyse the normal force distribution on the contact area of a sessile droplet in the static state and during the vibration. Next, we show that the viscosity (from ~1-30 mPa s) can be estimated within an error of less than 10% from the attenuation rate of the cantilever output during the tapping-induced vibration of small droplets (~2.4 μL). In addition to the advantage of the small sample volume, the proposed viscometer has simple operation and readout schemes, which are desirable for many applications, including point-of-care testing and drug development.