| The liquid metal is liquid at room temperature, characterized by good electrical conductivity, thermal conductivity and liquidity. The liquid metal has been widely used in a great many areas. With the constant development of MEMS technology and the constant improvement of advanced manufacturing technology, the liquid metal has been used as the contact material of electrical switch in the area of MEMS. Some problems in solid-solid contact, such as contact wear, signal bounce and poor reliability, can be solved by using liquid-solid contact. The metal droplet in the micro inertial switch is used as electrical contact material and also used to sense the acceleration signal. The micro inertial switch based on metal droplet is characterized by small contact resistance, no moving parts, high reliability, large overload current, etc. The signal electrode can be closed through the flow of mercury droplet in varying cross-section microchannel under the inertia force. When the inertia force reaches the acceleration threshold, mercury droplet comes into the reservoir through the micro-valve to close the signal electrode. The capillary micro-valve is formed by varying cross-section microchannel. When mercury droplet moves through the micro-valve, the capillary force prevents mercury droplets from going through the micro-valve. Only when the mercury droplet reaches the critical state (acceleration threshold) under the inertial force, the mercury droplet can go through the micro-valve. Therefore, the micro-fluid inertia switch based on the metal droplet has good threshold characteristics. The dynamic characteristics of metal droplet in the varying cross-section microchannel is obviously different from that in the conventional channel and uniform section microchannel. At this moment, the surface tension, on-way resistance, viscous force, capillary force have become the main factors influencing the dynamic behavior. The dynamic characteristics of metal droplet is closely related to the acceleration threshold and response time of micro-fluid inertia switch. Both the dynamic characteristics of metal droplet and the acceleration threshold and response time of the switch are influenced by inertia force, structure parameters of varying cross-section microchannel, surface roughness and the external environment factors.With the background of micro inertial switch, the thesis studies the dynamic characteristics of metal droplet in the varying cross-section microchannel. The influence of inertia force, structure parameters of vaying cross-section microchannel, surface roughness and the external environment factors on the dynamic characteristics of mercury droplet, acceleration threshold and response time of switch are studied by the theoretical analysis, numerical simulation, optimization design and experimental measurement. The dynamic characteristics of metal droplet in the varying cross-section microchannel under multi-physics coupling are obtained. The research methods can provide important reference for the design and optimization of other new MEMS devices based on metal droplet. The research is conducted mainly from the following aspects:(1) According to the structure and working principle of micro-fluid inertia switch, the mechanical property of metal droplet under the inertial force, surface tension, viscous force, wall pressure and capillary force are studied by the statics and dynamics theories. Based on the Young-Laplace equation, the semi-analytical model framework of acceleration threshold is built. The Morris method is adopted to conduct the local and global sensitivity analysis for structural parameters of varying cross-section microchannel, and determine the key structural parameters that influence the dynamic characteristics of metal droplet.(2) The surface roughness usually significantly influences the dynamic characteristics of metal droplet. The influence of the surface roughness on the dynamic characteristics of metal droplet are considered during the structural design. Due to the complex structure and random distribution of actual rough surface, it is difficult to clearly describe the influence of actual rough surface on the dynamic characteristics of metal droplet. Therefore, the regular microstructure can be builded on the actual rough surface. By reasonably designing the micro structural shape and parameters, the wetting behavior of metal droplet on the regular microstructure is in the Cassie state. The contact angle hysteresis model of Cassie state is used to analyze the influence of regular microstructure surface on the dynamic characteristics of metal droplet. The method of regular microstructure makes the effect of surface roughness on the dynamic characteristics controllable.(3) Aiming at the computation efficiency and precision in the numerical simulation, the VOF-CLSVOF joint model is adopted to track the phase interface. The different phase interface tracking model are automatically invoked through Matlab software at various movement stages of the metal droplet. The VOF-CLSVOF joint model combines the advantages of VOF and CLSVOF model. The joint model can effectively improve the computation efficiency and precision. In numerical simulation, the VOF-CLSVOF joint phase interface tracking model, surface tension model, and dynamic contact angle hysteresis model are adopted to study the influence of key structural parameters of varying cross-section microchannel, metal droplet volume, external environment factors on the dynamic characteristics. Based on the numerical simulation results of quasi-static acceleration threshold and response time, the orthogonal test is adopted to identify the semi-analytical model framework of acceleration threshold. The regression orthogonal design method is adopted to establish the regression model of response time. Both models can lay the foundation for the optimization design of the key structural parameters of varying cross-section microchannel.(4) Based on the semi-analytical model of acceleration threshold and the regression model of the response time, the target acceleration threshold and robust performance are taken as the optimization goals, and the response time and range of key structural parameters are taken as the constraint conditions, the particle swarm optimization algorithm is adopted to optimize the key structural parameters of varying cross-section microchannel. A set of optimized parameters is obtained. Under the fixed metal droplet volume, the optimized parameters can meet the target threshold of switch with high robustness, and make the response time satisfies the constraint condition.(5) The process technique of vertical microchannel, control technique of uniform microchannel depth and the material compatibility design technique of metal electrode are studied in the paper. The switch samples are fabricated by ICP etching, metal sputtering, anodic bonding and other key technologies, and the metal droplet volume is adjusted by the regulation hole and regulation microchannel. The test results show that the switch acceleration threshold decreases with the increase of metal droplet volume, and the switch acceleration threshold is adjustable through the adjustment of metal droplet volume. Compared with the average test threshold, the maximum accuracy of acceleration threshold test result is0.6%and the minimum accuracy is2.3%. The accuracy results show that the micro-fluid inertia switch has good performance stability. As the test results are consistent with the theoretical analytical results and numerical simulation results, the validity of the semi-analytical model and numerical simulation model are proved.(6) Finally, as an exploratory research, the dynamic characteristics of metal droplet in the varying cross-section microchannel under the inertial-electrowetting coupling are studied. Aiming at the target acceleration threshold, the tolerance analysis for the key structural parameters of varying cross-section microchannel and the metal droplet volume are conducted. The applied voltage adjusts the contact angle hysteresis of metal droplet in the microchannel, and the acceleration threshold of micro-fluid inertia switch is further adjustable. This method provides another alternative option for threshold adjustment.To sum up, with the application background of micro inertial switch, the dynamic characteristics of metal droplet in varying cross-section microchannels under multi-physics coupling action are studied by theoretical modeling, numerical simulation, optimization design and experimental test in the paper. The VOF-CLSVOF joint model is adopted to trace the phase interface, and the relationships between the switch threshold acceleration and response time with the key structural parameters of varying cross-section microchannel are obtained. The particle swarm optimization is used to optimize the key structural parameters of varying cross-section microchannel. The paper also studies the micromachining technology and metal droplet volume tuning technology. The acceleration threshold can be adjustable by tuning the droplet volume. |
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