The mechanical, electrical, and thermal properties of two-dimensional materials such as graphene depend strongly on their applied strain and on the out-of-plane wrinkles and defects present in freestanding membranes. Quantifying the relationship between strain and structure requires applying a controlled load to a sample while simultaneously imaging it at high resolution, which is only achievable with a tensile stage small enough to operate inside an electron microscope. This thesis characterizes two such microelectromechanical (MEMS) devices, both designed for in-situ TEM tensile testing of micro- and nanoscale samples: an electrostatic device based on a comb-drive actuator, and an electrothermal device based on chevron thermal actuators. Both apply a uniaxial strain to a sample mounted across a gap and sense the resulting force through a capacitive comb structure. For each device, the actuation, sensing, and operational limits were described analytically as functions of geometry, material properties, and actuation input, and the models were compared against experimental measurements obtained through SEM imaging, Raman thermometry, and capacitive readout.

For the electrothermal device, a four-parameter model was fitted jointly to displacement and temperature data and reproduced both datasets well, with each fitted parameter retaining a physical interpretation. For the electrostatic device, the predicted pull-in displacement of 0.61 μm, shown analytically to depend only on the initial comb geometry, was confirmed experimentally, while the capacitance measurement was dominated by parasitics and could not be resolved. Two production-related effects shaped the results: the omitted molybdenum conductive layer, which caused the surrounding chip to heat substantially during thermal actuation, and incomplete deep reactive-ion etching, which restricted functional testing to the 5 μm devices.

The two devices are found to be complementary rather than competing, each covering part of the intended measurement space, and the most impactful improvements for future work are identified as incorporating the molybdenum layer, fabricating functional 20 μm ... Read more

Imaging below sample-surfaces non-destructively at the nanometre scale is challenging. Overcoming this challenge will enable applications such as 3D imaging of computer chips or characterising adhesion between materials. A promising candidate is to combine Atomic Force Microscopy (AFM) with GHz photoacoustics (PASSAFM). AFM is capable of lateral resolutions in the nanometre range due to its tip-size, but cannot image three-dimensionally. Ultrasound propagates non-destructively through any material with a depth resolution proportional to the acoustic wavelength, but the lateral resolution is in the micrometre range for high-tech applications.

The AFM probe guides the acoustic waves, focussing the sound into the tip. The amount of focussing and acoustic transmission depends on the shape of the AFM probe, especially at the tip. This thesis investigates the influence of tip-diameter and geometry of nine different AFM probes. We observe nine discernable acoustic signals, following similar trends for frequencies between 40 GHz and 85 GHz. We show that the relation between tip-diameter and acoustic focussing follows a linear scaling trend. ... Read more

Acoustic levitation provides a promising method for contactless transportation of objects in air. This opens opportunities, particularly in high-precision industries where mechanical contact is undesirable. For acoustic levitation to serve as a viable alternative, it is necessary to investigate the extent to which the operating speed can be increased while maintaining precise control.
Therefore, this thesis investigates the properties on which the acceleration and velocity limits depend and experimentally evaluates their influence. The study consists of two parts: an acceleration analysis and a velocity analysis.

In the first part, the maximum stable acceleration is set by the acoustic trap stiffness and expressed in terms of the measured eigenfrequency \(f_0\) and the wavelength \(\lambda\). Experiments are conducted on spherical particles of varying diameters (0.593 to 2.886 mm) and densities (30 to 2500 kg/m\(^3\)), driven sinusoidally at sub-resonant frequencies. The measured acceleration limits are compared with theoretical predictions based on a linearised small-particle (Rayleigh-limit) force description and a Taylor-expansion-based model. For most particles, the Taylor-based formulation reproduces the experimental acceleration limits within \(\pm 15\%\) in both directions. In contrast, the linearised model systematically overpredicts the achievable acceleration, with typical deviations of 40-50\% in the vertical direction and similar overestimation in the horizontal direction. The remaining discrepancies are attributed to cross-axis coupling and gravity-induced equilibrium offsets.

In the second part, the maximum achievable velocity is studied through a balance in forces between the acoustic radiation force and aerodynamic drag, using a nonlinear drag model. Velocity tests are performed using sinusoidal excitation with gradually increasing amplitude. The measured velocities are compared with analytically derived predictions and dynamic simulations. While agreement is good in the horizontal direction, the vertical direction shows larger phase lag and earlier loss of stability, indicating increased damping and nonlinear effects at high speeds.

The results demonstrate that maximum acceleration scales with \(f_0^2 \lambda\) and is direction dependent. Another result is that the particle density and diameter influence performance via eigenfrequency and drag. For acceleration, the Gor'kov-based model shows reasonable agreement, whereas for velocity, a Reynolds number-dependent increase in damping is observed, particularly in the vertical direction.

This work provides a validated experimental methodology for characterizing acceleration and velocity limits in acoustic transport, contributing to the design of faster and more reliable non-contact handling systems. ... Read more

Piezoelectric vibration energy harvesters (PVEHs) represent a promising power source for low-energy wireless sensors. However, their performance degrades significantly when ambient vibration frequencies deviate from the harvester resonance. Bi-stable concepts can broaden the operational bandwidth through nonlinear dynamics, yet a quantitative understanding of how boundary condition variations affect resonance tuning in post-buckled harvesters remains limited. This thesis investigates the use of controllable boundary conditions to tune the eigenfrequency of a nonlinear, bi-stable piezoelectric vibrational harvester, with particular focus on the symmetric clamp angle and axial pre-tension.
A combined numerical-experimental methodology is developed. A parameterised multiphysics finite-element model implemented in COMSOL captures the post buckled equilibrium configuration, piezoelectric coupling, and the linearised frequency response under base excitation. In parallel, a dedicated experimental platform, referred to as the Green Gobbling device, enables repeatable adjustment of the clamp angle and axial pre-tension. This experimental setup is supported by a comprehensive validation study to ensure that the measured resonance shifts are reliable.
Both numerical simulations and experimental results demonstrate substantial and nonlinear tuning capability. Numerically, increasing the clamp angle from 3◦ to 8◦ shifts the primary resonance from 75.44 Hz to 49.28 Hz, corresponding to a reduction of approximately 35%. Similarly, increasing the axial pre-tension from 0.25% to 0.50% raises the resonance frequency from 58.93 Hz to 69.94 Hz, representing an increase of approximately 19%. Experimental results confirm the same qualitative trends, with even larger relative shifts observed. For instance, the base well resonance decreases from 80 Hz to 35 Hz, approximately −56%, across the clamp angle sweep and increases from 42.7 Hz to 68.5 Hz, approximately 61% across the pre-tension sweep. In all cases, the top well configuration consistently exhibits lower resonance frequencies than the base well configuration.
Overall, this work provides experimentally supported insight into boundary condition driven resonance tuning in bi-stable PVEHs and establishes a foundation for future adaptive tuning strategies aimed at robust energy harvesting under variable excitation conditions. ... Read more

In this thesis, a femtosecond pump-probe laser setup is used to study GHz acoustic waves in suspended phononic crystal (PnC) waveguides. These membranes consist of 200 nm thick Si3N4 coated with a 20 nm gold film and shamrock shaped PnCs. Various COMSOL simulations of these waveguides are conducted to investigate the expected behavior of the membranes. These simulations not only reveal the eigenmodes and phononic bandgaps of two different membranes, but also show the formation of a 300 MHz Lamb wave inside the waveguide. Experimental results show that this 300 MHz Lamb wave is confined inside the waveguide, when the frequency of the Lamb wave falls within the phononic bandgap of the membrane. For membranes with a bandgap that does not include the frequency of the Lamb wave, propagation through the PnC lattice is seen. Finally, an Acousto Optic Modulator (AOM) is successfully integrated in the pump-probe laser setup to increase the frequency resolution of the measurements. The experimental data obtained with this higher frequency resolution shows similar results of wave confinement and proves repeatability of the measurements. ... Read more

There is demand for non-invasive, low-cost and accurate sap flow measurement in horticulture in order to optimize yield, heating and irrigation. The current state-of-the-art of sap flow technology and ultrasonic sensors do not fulfil this research gap. The novel concept for a non-invasive ultrasonic sensor for slow flow rates demonstrated by a TU Delft bachelor thesis group was a promising alternative solution. To find out if the novel concept could be developed in to a reliable alternative method for sap flow or slow flow rate measurement this thesis project sought to answer the following research question: What are the design sensitivities and parasitic sensitivities of the non-invasive ultrasonic flow sensors accuracy and how can the latter be mitigated. The experimental setup of the previous research was constructed and improved upon. A one-dimensional analytical model of the setup was composed to evaluate the contribution of the transducers oscillator dynamics to the output measurement. This research unfortunately found that the sensors seemingly high resolution to very small flow rates can be attributed to a consistent measurement error. ... Read more

Within ASML’s extreme ultraviolet (EUV) lithography machines, a photomask defines the pattern on the silicon wafer. To protect the photomask from particles that are shed from inside the machine and to ensure the quality of the printed patterns, a highly EUV-transparent pellicle is suspended in front of the photomask. Various materials have been explored to meet the stringent requirements for such pellicles. One potential material is a random carbon nanotube (CNT) network, due to its favorable mechanical and thermodynamical properties. Despite the high EUV transmission rate of around 90%, the 10% of absorbed EUV radiation can lead to a temperature increase of the pellicle on the order of several hundred kelvin. It is essential to remove this heat as quickly as possible to prevent the pellicle from becoming thermally instable. Therefore, a thorough understanding of the thermodynamics of CNT pellicles at elevated temperatures - up to around 900 K - is required. In this thesis, the optomechanical method is successfully applied to extract key thermal properties such as the thermal expansion coefficient, specific heat capacity, and the thermal conductivty of 2D CNT networks up to 1300 K. ... Read more

The industry has continuously downscaled integrated circuits onto a silicon base plate, now mass producing microchips at sub-millimetre scale. As adhesive forces grow at this scale, methods of non contact handling are becoming increasingly more interesting for transporting the chips. Currently, the most promising form of non-contact handling is acoustic levitation. In this approach, cuboid shapes can be manipulated in six degrees of freedom using multi-axis acoustic levitators. Levitated objects exhibit low stiffness and low damping, making the system vulnerable to disturbances. To enable precise positioning of a cuboid chip, feedback is required to counteract these disturbances. Feedback could potentially be implemented using a low-latency tracking algorithm.
This work focusses on implementing an event camera to track the position of an acoustically levitating cuboid within a multi-axis levitator. The Metavision 3D Detection and Tracking algorithm is used to perform the tracking. First, the illumination and surface properties of various concepts are investigated to generate events along the edges of the cuboid on the event plane, while minimizing events on the cuboid’s surface. These aspects have been identified as essential for the effectiveness of the algorithm. Second, the conditions under which the tracking algorithm can be applied are examined. These include both static and dynamic conditions, failure velocity, and influence of various parameters.
The findings from these investigations demonstrate that the Metavision tracking algorithm is capable of reliably tracking a levitating cuboid in stationary conditions with multiple high-intensity diffuse light sources using an event camera, ensuring that the tracking parameters are specified within certain bounds. However, when the cuboid is actively actuated, the algorithm becomes unreliable. Frequent tracking failures are observed under these dynamic conditions. These results contribute to the broader concept of implementing feedback for acoustically levitating cuboids in potential future applications. ... Read more

This thesis investigates the forces and torques acting on acoustically levitated cuboids, with the aim of improving translational and rotational control in acoustic transportation applications. Traditional trans port methods rely on mechanical pick-and-place systems, which introduce challenges such as fragility and contamination. Acoustic levitation presents a contactless alternative that eliminates these issues. To address the research question: “1: Can we develop a model to determine the stiffness and torsional stiffness values of acoustically levitating cuboids of arbitrary shape, in an arbitrary pressure field?” the study begins by reviewing the fundamental principles of acoustic levitation, including acoustic radiation force, the Gorkov potential, and phased array transducer (PAT) configurations. It then explores various modeling approaches for determining the forces and torques on levitated objects, comparing the strengths and limitations of Gor’kov-based, finite difference time domain (FDTD), finite and boundary element method (FEM/BEM), and a proposed simplified trapping model.
The findings show that while the simplified trapping model lacks predictive accuracy, a Finite Element Method (FEM) model was introduced as a more reliable alternative. The FEM model, using a sound hard boundary assumption, agrees well with prior models of forces on non-spherical particles, that cannot be predicted with Gorkov’s or King’s method, and its implementation in Comsol makes it more accessible to a broad audience. In addition, the model predicts accurately the torques that act on non spherical particles and for the first time such a model is experimentally verified for all three translational and three rotational degrees of freedom....
... Read more

Piezoelectric energy harvesters convert vibrational energy into electrical energy, thereby reducing the dependence of wireless devices on batteries. However, most existing harvesters have a limited bandwidth, generating usable power only within a narrow range of excitation frequencies. Buckled beam harvesters address this issue by intentionally introducing nonlinear behaviour, but systematic design strategies for such devices are still missing due to the absence of versatile models and limited understanding of their dynamics.
In this thesis, these challenges are addressed by developing a model that combines finite element analysis with lumped-parameter equations and validating it experimentally. The resulting framework captures higher-order vibration modes and nonlinear dynamic phenomena such as secondary resonances and softening behaviour. It is demonstrated that these effects can be exploited to optimize the design and broaden the bandwidth of buckled beam piezoelectric energy harvesters. Based on the improved physical understanding, several practical design improvements are proposed. ... Read more

3D imaging of subsurface structures at the nanoscale is a longstanding challenge in microscopy. Ultrasound enables subsurface imaging, but nanometer depth resolution requires high-frequency sound waves, achieved by heating with pulsed lasers. Combined with Atomic Force Microscopy (AFM), which offers atomic-scale surface resolution a promising candidate for full 3D nanoscale imaging emerges.

This thesis focuses on the development of such a combined ultrasound-AFM system.
Novel optical detection techniques are introduced for both the ultrasound and the AFM, enabling simultaneous operation.

Acoustic waves up to 100 GHz are generated and detected, used to characterize the thickness and adhesion of thin films. We demonstrate that the conical AFM tip acts as an acoustic lens, focusing the wave into the tip apex. When in contact with a sample, changes in the reflection signal confirm acoustic transmission—opening the door to true 3D nanoscale imaging. ... Read more

This paper tests the effect of multi-frequency vibrations on the power output of piezoelectric
energy harvesters. These tests were conducted to show how the output power scales when an energy harvester is excited not only at its eigenfrequency, but also at an off-resonance frequency and how well theoretical models predict power output under these conditions. A linear cantilever and bistable buckled beam are tested experimentally on an electrodynamic shaker and simulated using existing models to compare the accuracy of these models under such excitations. The excitation signal has one fixed frequency component, which matches the eigenfrequency of the energy harvester; and a second vibration component, which has a frequency and amplitude that are varied. For a linear cantilever, it was shown that an existing closed-parameter model can predict power output for the mechanical and electrical parameters, where the power is a summation of the power generated by each separate frequency. However, even a simple cantilever shows nonlinearity for high excitations; this is something one needs to account for if you use the model to calculate expected power output.
For a nonlinear harvester it was shown that the power output scales linear with the amplitude of vibration when subjected to a single sine. For multi-sine vibrations, the power output is only slightly increased by adding the second frequency, indicating that the beneficial bandwidth of a nonlinear energy harvester is much smaller when absorbing energy from a second frequency. ... Read more

When a free-standing membrane is actuated with photothermal method, the heat flux requires a certain time to diffuse through this membrane. This duration of time, called thermal time constant, is important for its application in sensors, nano-electromechanical systems, filters, etc. This report is devoted to exploring what plays a major role in the variation of experimentally measured thermal time constants, and to investigating the relationship between dumbbell dimensions (namely, two drum radius R1 for drum 1 and R2 for drum 2, half bridge width y0, bridge length x0) and thermal time constants.

First, dumbbell resonators of various dimensions were fabricated using exfoliated molybdenum disulfide flakes. Optomechanical experiments were conducted on these devices, involving two collocated (actuation and measurement located in the same drum) and two non-collocated (actuated at one drum and measured at the other) measurements for each device. Accordingly, four thermal time constants were extracted for each resonator through curve fitting. To understand temperature distribution and experimental variation, a COMSOL model and an analytical model were established, solving the heat equation with a harmonic laser actuation.

As a result, four thermal time constants for 16 devices were extracted. These experimental data were verified with a synergy of the two models. The primary contributors for large experimental data variation were the 2D material irregularities and laser locations. For collocated τ, it was almost unaffected by dumbbell dimensions, except for R1 which gave a parabolic curve. For non-collocated τ, it increased monotonously with x0, but a minimum was always observed when sweeping the other three parameters.

Such minimum occurred when x0 is around 25% of the drum radius. This minimum was attributed to a balance between the efficiency of heat transport across the bridge and the acceptable duration required to heat the bridge itself. Meanwhile, the COMSOL model and the analytical model disagreed on the relationship between non-collocated τ and y0, R1, R2. Moreover, the analytical model’s deviation from the COMSOL model increased with larger bridge width or thermal conductivity. This stemmed from errors in the assumed boundary conditions: the existence of the bridge altered the temperature distribution at the boundaries of drum 1, and in the analytical model the boundaries of the dumbbell are fixed while in the COMSOL model they are controlled by the substrate. ... Read more

systems. Miniaturization creates challenges in handling delicate, small components, rendering traditional methods inadequate, necessitating an alternative. Acoustic levitation, which uses ultrasound waves to suspend objects without physical contact, offers a promising solution for containerless transportation. This technology can accommodate various materials regardless of their electromagnetic or optical properties and eliminates particle generation due to friction. Applications span across chemistry, biology, and high-tech industries, enabling manipulation and assembly of small components. However, acoustic levitation is highly sensitive to environmental factors such as temperature and air currents, which can compromise stability. Most studies have primarily focused on understanding the dynamics of acoustic levitation and developing models to describe the underlying physical phenomena. This study shifts towards actively controlling them by adding artificial damping. To achieve this, this thesis introduces the use of event cameras, which capture high-speed movements without redundant frame data, making them highly efficient. The goal is to create a control feedback loop to correct for disturbances and improve the step response of acoustically levitated objects. The methodology involved integrating an event camera into the acoustic levitation setup and implementing a tracking algorithm to track the position of a levitated object. System identification was performed, revealing higher-order harmonics and allowing the development of a second-order model of the levitated object. This model was used to design a feedback controller integrated with the event camera. Experimental results demonstrated a substantial improvement in system performance. The bandwidth of the system improved to 40.8 Hz, the settling time decreased from 10 seconds to 0.8 seconds, an improvement by a factor of 12.5. Additionally, the steady-state error was significantly reduced, and the introduction of artificial damping successfully eliminated the first resonance peak, though a second higher-order resonance peak remains. This work highlights the potential of using event cameras in control loops for acoustic levitation, representing a significant step towards more precise control of levitated objects. ... Read more

This study explores the design and implementation of an electrowetting-on-dielectric (EWOD) system for precise die positioning in a chip assembly environment, utilizing a millimeter-scale ”chip-on-droplet” approach. Two methodologies were investigated: coarse positioning using digital signals, a method well-documented in prior research, and fine positioning using analog signals, which, to the best of our knowledge, had not been applied to droplet positioning at the time of writing. The primary advantage of the fine positioning approach lies in its independence from feature size, allowing for significantly smaller step sizes. Experimental results demonstrated that the analog system achieved step sizes as small as 3.5 μm in a ”droplet only” setup, outperforming the ”traditional” coarse positioning system. Performance limitations were found to arise primarily from the tracking setup rather than the EWOD system itself.
The study also evaluated various surface layers, confirming prior findings that a smooth, thin, and homogeneous surface layer is critical for achieving consistent and precise movement. Attempts to en- hance droplet guidance through modified surface structures were unsuccessful due to pinning forces exceeding the generated EWOD forces. These findings highlight the potential of analog signal-driven EWOD systems for fine positioning applications, with important implications for chip assembly tech- nologies. ... Read more

Electric and hybrid vehicles are a common sight on the roads these days. The electric machine is known to be quiet, but sometimes high-frequency sounds are observed when driving. Over the years many different solutions have been developed to make the electric machine quieter, but almost no research looks into the possibility to generate sounds. This thesis looks into the possibility to produce nice sounds and reduce annoying sounds with electric machines. This is done by introducing harmonics into the current and creating additional excitations of the structure. To gain a complete understanding of this topic the electromagnetic domain and the structural domain have been analysed. A mathematical description of the forces has been created to be able to make predictions about the forces created by an injected current harmonic. Using the electromagnetic model the exact influence of current harmonics on the forces has been verified. The sound power level produced by the forces acting on the structure is calculated using the structural model. By properly selecting the current properties it is possible to create new temporal orders that produce sounds. For the analysed electric machine the sound power level of the desired sound is on average approximately 20dB lower than the sound power level of the fundamental current. Furthermore the sound power level is dominated by the excitation of the breathing modes. Using current injection existing sounds can be reduced. Using brute force optimisation the average radial forces were reduced. This reduces the sound power level during most of the run-up and reduces the influence of the breathing mode by up to 11dB. At some points during the run-up more sound is produced, because the tangential forces are increased, while the radial forces are decreased. In the end it is shown that the influence of the introduced current harmonics on the acoustics can be analysed using mathematics and the simulation results. New sounds can be produced, but the sound power level is limited by structural design of the electric machine. ... Read more

Nanomechanical resonators made of two-dimensional (2D) materials are the subject of intensive research due to their remarkable properties, allowing them to operate at high frequencies with high sensitivity. However, dissipation losses and manufacturing issues have prevented them from reaching their full potential. This thesis aims to overcome these challenges by dry-transferring 2D materials onto a MEMS and clamping them using electron beam-induced deposition. By in-plane straining the membranes using MEMS, the tensile energy is increased, thereby diluting intrinsic losses. This approach increased the Q-factor of 2D material resonators by 91% and allowed measuring forces down to sub-piconewtons, outperforming commercially available silicon-based force sensors. ... Read more

Photonics biosensors convert biomolecular interactions into quantifiable optical signals for biomedical analysis, which enable continuous monitoring of health indicators. Among them the microring resonator has a good sensing performance and a very broad application prospect. This thesis studies sensing with microfluidic integrated microring resonator photonic microchips.
This thesis adopts finite element method to simulate the optical behavior of waveguides and reactions in the microfluidic channel. The microfluidic channel was designed and prepared and then integrated to the photonic chip. The optical performance parameters of the micro ring resonator were tested by using a high-precision optical test system. The sensing performance of waveguide microring was studied using different aqueous solution as the detection object. The feasibility and effectiveness of the optical waveguide chip sensing have been preliminary verified.

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