Energy dissipation in hollow beam resonators in function of microfluidic channel off-axis placement

Suspended Microchannel Resonators (SMRs) are hollow resonant structures containing an embedded u-shaped micro fluidic channel. Having the sample fluid confined inside the resonator allows for real time detection of liquid compounds while the device quality factor remains almost unaffected. Theoretical studies and experimental results have shown that these devices present a nonmonotonic energy dissipation as the fluid viscosity is increased or decreased. This is in contrast with conventional cantilevers immersed in fluid, where the quality factor monotonically decreases as the fluid viscosity increases. In the simplest configuration, the channel contained inside the cantilever is perfectly centered about the neutral axis of the resonant structure. Analytical models have shown variation of the device quality factor by several orders of magnitude when the microfluidic channel is placed out of the beam neutral axis. This project aims to experimentally demonstrate the relation between SMRs’ energy dissipation and off-axis placement of the microfluidic channel, in function of fluidic properties as compressibility and dynamic viscosity. The student’s tasks are the following: • Review and understand the state of the art about theoretical studies concerning energy dissipation of hollow beam resonators; - Propose an experimental plan presenting interesting geometries and fluidic properties to be tested; - Design and fabricate samples to be tested; - Optimize an experimental setup; - Collect experimental results and compare with theoretical studies found in literature.

PZE-transduced Suspended Microchannel Resonators for sensing applications

Annalisa De Pastina

This PhD thesis aims at developing a system which can measure the mechanical properties of fluidic samples in the picoliter range. The ultimate goal is the characterization of cancer cells and viscoelastic fluids (i.e. biological fluids), in order to study their mass and stiffness for diagnostic applications. The core of this project is the design, fabrication and characterization of hollow micromechanical resonators containing embedded microfluidic channels and provided with integrated piezoelectric (PZE) transduction. The measurement strategy consists in monitoring the device resonance frequency over time, while the analyte flows through the microchannel. Arrays of singly-clamped and doubly-clamped resonators are made of low stress Silicon Nitride (ls-SiNx) and vibrate in a frequency range from tens of kHz to few MHz. Microfluidic channels span 25%, 60% and 100% of the cantilever length in order to compare the resonance frequency shift induced by the same particle at different positions, along identically-designed beams. The goal is to disentangle analyte mass and stiffness contribution from the suspended microchannel resonator (SMR) mechanical response. The cross section of the suspended fluidic channel, 6 um x 10 um, is designed to measure particles in the same size range (i.e. red blood cells or circulating tumor cells). SMR energy dissipation is studied via FEM simulation, as a function of fluidic properties and geometrical dimensions. A coupled fluid-structure interaction (FSI) problem, based on linear elasticity and linearized Navier Stokes equations, is studied. The model is found well in agreement with theory and experiments presented in literature, considering the viscosity range of interest in case of biosensing application. The fabrication of transparent and PZE-transduced suspended microchannel resonators is achieved via a 6-mask process flow. 250, 500, 750 and 1000 um long singly- and doubly-clamped SMRs are fabricated. Microfluidic channels are defined via the etching of high aspect ratio-trenches through a sacrificial layer of polysilicon, and filled with ls-SiNx. Discontinuous etch apertures are defined on top of the buried fluidic network and enable channel emptying in 25 minutes via KOH etching. PZE electrodes in platinum and aluminum nitride are deposited via sputter deposition and provide independent actuation and readout of each resonator, for the first time in SMR arrays. The development of an experimental platform enables fluidic and electrical connections, as well as temperature control during devices characterization. The mechanical response of fabricated sensors is measured in air and vacuum environment, while channels are filled with air, water and Isopropyl Alcohol. PZE electrodes demonstrate efficient actuation, in the order of 1.5 nm/V for 250 um- long singly-clamped SMRs. However, PZE detected signals result very low in amplitude, thus optical detection via laser Doppler vibrometer is preferred. Frequency stability of fabricated sensors is studied in order to estimate their sensing performances and characterize their physical limitations. 250 um-long SMRs filled with DI water exhibit a frequency stability of 30 ppb at 400 ms integration time, in air. This result is in agreement with values found in literature for SMRs sensors and translates in an estimated mass sensitivity of few femtograms.

EPFL2018

Moisture and energy dynamics in fields soils

Francesco Ciocca

Soil moisture is crucial to water-cycle as it affects infiltration, runoff and land-atmosphere interactions. In dry soils the role of water vapor fluxes becomes essential and causes a tight coupling between soil moisture dynamics and heat transfer, which makes measurements and modeling extremely difficult. Since advances in both theory and experimental techniques are required, we have focused on both aspects: we have performed theoretical-numerical investigations of soil moisture and energy dynamics at the land surface; tested novel measurements techniques, in laboratory and at field scale, to provide new insights into soil moisture dynamics and contribute to a comprehensive theory of coupled energy and moisture transport. As the dynamics of physical systems are dictated by conservation equations, it is common practice to estimate quantities that cannot be directly measured (e.g. vapor fluxes) from the residuals of balance equations. In laboratory as well as in field experiments, residuals are calculated from measurements that are sparse in space and discrete in time. We have theoretically and numerically shown how the use of discrete data may lead to large residuals even without measurements or model errors. We have demonstrated that residuals are very sensitive to the distance between the measurements and that they increase in presence of nonlinear processes, such as moisture transport. A careful error analysis is required to assess the reliability of residuals computed from discrete data and avoid misinterpretations of the underlying physical processes. Another crucial element dictating soil moisture dynamics and evaporation into the atmosphere is soil water-retention. Popular parametric models of the retention curve present a matric potential that becomes infinite at residual saturation, which is the water content below which liquid flow stops. Few extensions of the water-retention curves have been proposed, which remain rarely used in practice. We have studied the effects of different models on liquid and vapor transport. We have shown that parameterizations that allow vapor flux below residual saturation yield larger vapor fluxes that result in a drier soil surface, whereas deep soil remains relatively wet. When diurnal forcing is considered, potential evaporation is reduced in the energy-limited regime and enhanced in the moisture-limited regime. We have established that in this case extended water-retention models predict larger heat fluxes that might critically impact models at whole scales (possibly including global circulation models). Discrimination between different theoretical descriptions and parametric models requires experiments that monitor moisture and energy dynamics. This demands the use of accurate sensors and measurement techniques that are able to simultaneously monitor several quantities (e.g. soil moisture, temperature, soil thermal properties, solute concentration in soil water). As in general different probes are used, problems arise due to different footprints and sampled soil volumes. Multi functional heat pulse probes (MFHPPs) have been conceived to overcome these issues by allowing simultaneous measurements of all required variables. We have assessed MFHPPs ability to measure ground heat flux, which has to be accurately known to avoid significant imbalances in the surface energy budget. We have employed an experimental setup able to generate a non-uniform heat-flux field and we have demonstrated that MFHPPs can reliably capture the magnitudes and directions of the fluxes. This allows more complete information on ground heat flux than that obtained by the classic probes (e.g. ground heat flux plates). To monitor soil state in large-scale field applications we have investigated the possibility to infer distributed thermal conductivity and soil moisture profiles with an optical fiber. Information is obtained by monitoring soil response to an active heat pulse generated from the metal shield armoring the optic cable, and analyzing the temperature differences of the cooling phase, monitored each meter by a distributed temperature sensing system (DTS). We have shown that soil moisture measurements inferred from soil thermal response (recorded by the DTS) are in good agreement with independent measurements in wet and intermediately wet soils; however in dry soils moisture content is underestimated and further investigations are required to improve this technique for dry conditions. In this study we have significantly improved the knowledge of coupled energy and moisture transport on both theoretical-numerical and experimental sides. We have proven that reliable measurements can be obtained at different spatiotemporal scales, and provided both prognostic and diagnostic methods to optimize their use. Finally, we have suggested crucial improvements to numerical models which might have implications beyond field-soils dynamics.

EPFL2013

3D FEM dissipation model of suspended micro channel resonators

Annalisa De Pastina, Andrea Fani, François Gallaire, Luis Guillermo Villanueva Torrijo

Suspended micro channel resonators (SMCs) consist of hollow resonant structures containing an embedded u-shaped micro fluidic channel. The confinement of the fluid inside the resonator allows real time detection of liquid compounds in air or in vacuum, while the device quality factor remains almost unaffected. Quality factor of conventional cantilevers immersed in fluid monotonically decreases as the fluid viscosity increases. On the other hand SMCs exhibit a non-monotonic energy dissipation as the fluid viscosity is increased or decreased. In this work we present a fully three dimensional modal analysis of the fluid structure interaction (FSI) problem to predict the quality factor dependence on fluid properties. The reliability of our model is demonstrated through literature comparison of analytical model and experimental results.