Study on a Novel Thermoelectrical Micro-Droplet Sensor

Abstract

Due to the limited functionalities and considerable expense of the currently prevailing principles for characterizing tiny drops W. Lang et al. proposed a groundbreaking method through observing the thermal behaviour of a concerned droplet upon its impinging upon a thermoelectric micro-flowmeter. M. Maiwald et al. have experimentally proved the feasibility of this proposal. This submitted thesis exhibits both theoretical explanation of the multi-region multi-phase process and technical aspects of development of a novel thermoelectric micro-droplet sensor.With low Weber numbers (c.a. 11) of the tiny drops, deformation and advancing of droplets are neglected. The final diameter of the meniscus is determined with help of Overall Energy Balance approach. Considering the temperature profile of the underlying membrane, one can mesh the meniscus in slices parallel with the heater, then apply the extended general heat conduction equation to the slice element. Based on the assumptions that the droplet in quiescent atmosphere can be approximated with a spherical cap with base radius $b$ and contact angle $\alpha$, and the problem is considered as a quasi-steady type, the analysis starts with the energy equation of a differential volume in the vapour mixture above the evaporating splat. The analytic models following two vaporization scenarios are compared with the data acquired by M. Maiwald et al. By introducing the assumptions that the liquid is incompressible and Newtonian, and the role of gravity/gravitation can be ignored, Navier-Stokes equations are applied to the infinitesimal volumes inside the droplet. Solutions to the Navier-Stokes equations gain a more precise insight into deformation and spreading of the droplet. The general heat conduction equation governs the temperature distribution across the liquid and the membrane. As to evaporation, the Clausius-Clapeyron saturation relation can be adopted to calculate the phase transfer. These phenomena are coupled through liquid mass, contact surface, and temperature-dependent parameters. With proper boundary conditions and initial conditions, numerical calculations were carried out, where free surfaces were tracked using Volume of Fluid algorithm together with the Fractional Area/Volume Obstacle Representation method.The confirmed analytic modelling techniques have been finally used to analyze the novel thermoelectric microdropletsensor. Analysis of the heat transfer and the phase transition of concerned picodroplets reveals the correlation between the evaporation rate and the droplet diameter.The thermoelectric microdropletsensors in four layouts have been fabricated mostly using standard thin film technology and micromachining techniques. The thermopiles constitute of PVD $\mathrm{W_{0.1}Ti_{0.9}}$ and in-situ LPCVD p-doped poly-Si. The silicon nitride membrane was through DRIE realized. The manufacture procedure of the sensors will be introduced in this thesis. In an effort to characterize the microdropletsensor, a measurement system has been developed and installed at IMSAS. The complete setup comprises picodroplet generating unit, stroboscopic module, and signal acquisition as well as monitoring component. The laboratory configuration will be provided in detail. Sample experimental results will be displayed as well. Finally, the vital conclusions of this thesis are summarized and some suggestions for the application of such microdropletsensors and improvement are presented.