Department of Mechanical Engineering

The vast capabilities of autonomous underwater vehicles (AUVs)—such as in assisting scientific research, conducting military tasks, and repairing oil pipelines—are limited by high operating costs and the relative inaccessibility of power in the open ocean. Wave powered AUV charging stations may address these issues. With projected increases in usage of AUVs globally in the next five years, AUV charging stations can enable less expensive and longer AUV missions. This paper summarizes the design process and investigates the feasibility of a wave powered, mobile AUV charging station, including the choice of a wave energy converter and AUV docking station as well as the ability to integrate the charging station with an autonomous surface vehicle. The charging station proposed in this paper meets many different commercial, scientific, and defense needs, including continuous power availability, data transmission capabilities, and mobility. It will be positioned as a hub for AUV operations, enabling missions to run autonomously with no support ship. The potential market for this design is very promising, with an estimated $1.64 million market size just for AUV technologies by 2025.

Computed Axial Lithography (CAL) represents a significant advancement in the emerging field of Volumetric Additive Manufacturing (VAM). CAL addresses key limitations of traditional photopolymer additive manufacturing technologies, by eliminating the need for layering and support structures. Unlike conventional methods, CAL prints components by illuminating all points within a desired geometry simultaneously, using tomographic reconstruction to form the object in a single step. This unique approach eliminates the relative motion between the object and the precursor material, enabling faster printing speeds and reducing the waste associated with support structures. However, CAL parts require post-processing steps before they can be utilized. CAL's core attributes make it particularly suited for In-Space Manufacturing (ISM), due to its fast fabrication times, wide breadth of materials it can use, and minimized footprint. CAL has been successfully demonstrated in microgravity during parabolic flight experiments. However to fully validate and understand CAL's behaviour in microgravity, all manufacturing and post-processing steps must be integrated. In June 2024, we conducted SpaceCAL Mission 3, testing this entire workflow on a suborbital flight aboard Virgin Galactic's SpaceShipTwo. During ∼140 s of microgravity, the system autonomously manufactured and post-processed four parts using PEGDA700 resin. Post-flight analysis showed that 2/4 parts were recognisable, while others were distorted due to bubble formation from residual water droplets, off-axis optical aberrations, and non-uniform solvent rinsing. Despite these limitations, this study represents the first integrated CAL workflow in space, providing an initial experimental demonstration and analysis for closed-loop in-space manufacturing.

Drawing inspiration from the panther chameleon's sophisticated color-adaptive abilities, we introduce a unified design framework for a new class of bio-inspired mechanochromic materials with tailored reflectivity across the electromagnetic spectrum. While structural coloration in nature relies on the mechanical adjustment of photonic crystal spacing, engineering such systems is often hindered by the barreling effect, the lateral bulging of bulk materials caused by Poisson's effect. To address this, we developed a mechanical metamaterial substrate optimized through a genetic algorithm and modeled it using Timoshenko beam theory to ensure a uniform strain field during deformation. The proposed system features a two-dimensional hexagonal lattice of composite dielectric nanopillars positioned on top of a micro-architected substrate fabricated via Multiphoton Lithography (MPL). The nanopillars consist of an MPL core coated with a high-refractive-index material to achieve complete optical band gaps. By dynamically adjusting the lattice constant through controlled, reversible elastic deformation, the reflected wavelengths can be shifted across broad spectral ranges. We demonstrate the scalability and robustness of this approach through three distinct structures tailored for the visible, mid-wave infrared (MWIR), and long-wave infrared (LWIR) regions. This methodology lays a foundation for scalable, reversible mechanochromic materials with significant potential for applications in adaptive camouflage, radiative thermal management, and wearable electronics.

The frequency response of pyroelectric sensors is fundamentally governed by thermal time constant (τ_th, determined by thermal mass and thermal conductance) and electrical impedance arising from film capacitance and readout circuit. Conventional bulk LiTaO₃ detectors are optimized for high responsivity at low modulation frequencies (0.1-10 Hz), possessing a large τ_th that thermally averages rapid temperature oscillations at elevated modulation frequencies, limiting fidelity in resolving dynamic varying thermal signals. Here, compositional and strain gradients are introduced into 100-nm-thick relaxor-ferroelectric films reducing τ_th to ≈2 µs and producing built-in potentials (≈1.45 V or 145 kV cm^-1) that enhance the pyroelectric coefficient and suppress the dielectric constant. This enables complementary dual-mode operation by enhancing current-mode electrical responsivity and improving the voltage-mode figure of merit - advantageous for superior temperature resolution (ΔT_min ≈ 30 µK). The responsivity peak shifts to near 1 kHz (>2500-times higher than conventional bulk sensors), with measurable responsivity extending to a carrier frequency of 100 kHz and amplitude-resolved detection at modulation frequencies up to 15 kHz. These results establish nanoscale internal-field engineering can reshape electro-thermal trade-off in pyroelectric thin films toward zero-bias, high-thermal-sensitivity, and amplitude-resolved thermal sensing across a wide frequency bandwidth.

High-fidelity optical propagation through the atmosphere is essential for free-space optical technologies, including laser-based remote sensing and optical communication. However, atmospheric turbulence severely distorts beams and compromises system performance. In this work, we employ hypergeometric-Gaussian (HyGG) vortex beams as probes to characterize and mitigate atmospheric turbulence. Using over 250,000 experimental and simulated frames, we show that refining the power spectrum density (PSD) can reduce numerical prediction errors by up to 79.8%. Concurrently, experimental observations supported by numerical simulations demonstrate that HyGG beams exhibit superior turbulence resilience across multiple metrics compared to conventional Gaussian beams, particularly in their ability to withstand over 5 times stronger turbulence while maintaining similar intensity fluctuations. These dual investigations, on both turbulence mitigation and robust beam solutions, converge to form a unified strategy for enhancing free-space optical system performance. Collectively, our findings provide new insights into light–turbulence interactions and highlight the practical utility of vortex beams under atmospheric conditions.

Passive evaporative cooling from a damp cloth or vessel can be used for localized cooling of perishables such as food and medicine, drinking water, and even people. Such applications have two key objectives, which are normally in tension with each other: minimizing the water consumption rate and maximizing the cooling (steady-state temperature swing between the cool cloth and the warm surroundings). Here, we present a simple device that delivers higher performance in both metrics simultaneously by adding a perforated aluminum foil sheet suspended over a stagnant air layer to thermally shield the evaporating surface while also facilitating mass transfer. Experimental results demonstrate a simultaneous 10%–25% increase in temperature swing and 2–3× reduction in water consumption rate as compared to conventional evaporative cooling. This improvement is achieved through careful modeling to optimize the coupled heat and mass transfer through the airgap (thermally insulating and vapor permeable) and perforated foil (infrared reflective and vapor permeable).

Abstract: Incorporating carbon dioxide (CO2) refrigerant into the heat pump (HP) technology can reduce climate change effects due to working fluid loss. However, CO2 refrigerants in an HP system may change from a gaseous state to a liquid or solid “snow” during the throttling and pressure-relief valve processes, which may block downstream flow in pipes and cause unsafe operation issues. This paper describes a Soave–Redlich–Kwong (SRK) statistical-thermodynamics-based model to predict CO2 refrigerant metastable state properties (enthalpy, temperature, and volume) exhibited during throttling through a leakage path or pressure relief valve in HP technologies. In addition, the simulation can predict changes in density fluctuations states exhibited during the process that can indicate an impending phase change and can predict exit state conditions for widely varying supercritical initial equilibrium states. Results indicate that when a constant enthalpy throttling line passes the freezing point temperature and approaches the spinodal line, the throttling process can result in sublimation of solid-state CO2 which may cause a blockage. Therefore, this simulation can be used to explore how changing initial state conditions, CO2 pressure ranges, and thermomechanical characteristics may result in the blockage of the pressure relief passage and unsafe conditions for users.

Ultrashort burst laser pulses are important for applications in laser processing and spectroscopy. However, existing burst generators face limitations in pulse time interval tunability and coaxiality. Here, we present a burst pulse generator that delivers coaxial ultrashort pulses with tunable pulse time intervals using a single translation stage. The constructed system produced eight femtosecond pulses with uniform intervals tunable from 100 fs to 1.93 ns, equivalent to burst-internal repetition rates spanning 10 THz to 0.52 GHz. We further demonstrated interval-dependent plasma emission in air filamentation, providing a compact platform to study burst-laser-induced phenomena.

Multifunctional materials that balance mechanical resilience and fluid dynamic efficiency are critical in engineering applications, yet their synergistic optimization remains challenging due to inherent trade-offs, computational expense, and high-dimensional design spaces. Inspired by the skeleton of the deep-sea sponge Euplectella aspergillum, this work presents an automated framework integrating Finite Element Analysis for mechanics, Computational Fluid Dynamics for flow behavior, and multi-objective Bayesian optimization. Leveraging high-performance computing, the framework efficiently explores complex design spaces to identify Pareto-optimal solutions. Optimized lattices achieve an average 140% increase in critical buckling load across a range of volume fractions relative to baseline designs, while simultaneously reducing drag, lift, and vortex shedding at porosities as low as 5%. We fabricate selected designs via stereolithography and validate them through compression experiments and particle image velocimetry, showing agreement with simulations. By jointly optimizing mechanics and fluidics, this work establishes a scalable methodology for designing lightweight, high-performance architected materials.

The Leidenfrost effect, best known as the formation of an insulating vapour layer beneath a liquid droplet that delays boiling, offers great potential for droplet manipulation and drag reduction. However, regulating the conventional Leidenfrost effect remains challenging due to the complex liquid dynamics involved. Here we report a behaviour that we term the capillary Leidenfrost effect, which enables stable and sustained solid levitation driven by liquid evaporation. It occurs at a temperature threshold that is below the Leidenfrost point of its droplet counterpart, yet without the need for specialized surface manufacturing techniques. Our structure is composed of periodically arranged capillaries that stabilize the liquid interface, enhance heat conduction and provide liquid storage capacities. The capillary Leidenfrost effect is generic in widely accessible natural materials and metals. Our experiments demonstrate the feasibility of long-distance, sustained self-propulsion and load delivery on a practical scale. This mechanism facilitates potential applications from contactless transportation and drag reduction to extended-distance delivery in harsh environments.