When fluids flow through straight channels sustained turbulence occurs only at high Reynolds numbers [typically $\text{Re}\sim O(1000)$ ]. It is difficult to mix multiple fluids flowing through a straight channel in the low Reynolds number laminar regime [$\text{Re}<O(100)$ ] because in the absence of turbulence, mixing between the component fluids occurs primarily via the slow molecular diffusion process. This Letter reports a simple way to significantly enhance the low Reynolds number (in our case $\text{Re}\le 10$ ) passive microfluidic flow mixing in a straight microchannel by introducing asymmetric wetting boundary conditions on the floor of the channel. We show experimentally and numerically that by creating carefully chosen two-dimensional hydrophobic slip patterns on the floor of the channels, we can introduce stretching, folding, and/or recirculation in the flowing fluid volume, the essential elements to achieve mixing in the absence of turbulence. We also show that there are two distinctive pathways to produce homogeneous mixing in microchannels induced by the inhomogeneity of the boundary conditions. It can be achieved either by (1) introducing stretching, folding and twisting of fluid volumes, i.e., via a horse-shoe type transformation map, or (2) by creating chaotic advection, achieved through manipulation of the hydrophobic boundary patterns on the floor of the channels. We have also shown that by superposing stretching and folding with chaotic advection, mixing can be optimized in terms of significantly reducing mixing length, thereby opening up new design opportunities for simple yet efficient passive microfluidic reactors.

Silicon-based kinetic energy converters employing variable capacitors, also known as electrostatic vibration energy harvesters, hold promise as power sources for Internet of Things devices. However, for most wireless applications, such as wearable technology or environmental and structural monitoring, the ambient vibration is often at relatively low frequencies (1–100 Hz). Since the power output of electrostatic harvesters is positively correlated to the frequency of capacitance oscillation, typical electrostatic energy harvesters, designed to match the natural frequency of ambient vibrations, do not produce sufficient power output. Moreover, energy conversion is limited to a narrow range of input frequencies. To address these shortcomings, an impacted-based electrostatic energy harvester is explored experimentally. The impact refers to electrode collision and it triggers frequency upconversion, namely a secondary high-frequency free oscillation of the electrodes overlapping with primary device oscillation tuned to input vibration frequency. The main purpose of high-frequency oscillation is to enable additional energy conversion cycles since this will increase the energy output. The devices investigated were fabricated using a commercial microfabrication foundry process and were experimentally studied. These devices exhibit non-uniform cross-section electrodes and a springless mass. The non-uniform width electrodes were used to prevent pull-in following electrode collision. Springless masses from different materials and sizes, such as 0.5 mm diameter Tungsten carbide, 0.8 mm diameter Tungsten carbide, zirconium dioxide, and silicon nitride, were added in an attempt to force collisions over a range of applied frequencies that would not otherwise result in collisions. The results show that the system operates over a relatively wide frequency range (up to 700 Hz frequency range), with the lower limit far below the natural frequency of the device. The addition of the springless mass successfully increased the device bandwidth. For example, at a low peak-to-peak vibration acceleration of 0.5 g (peak-to-peak), the addition of a zirconium dioxide ball doubled the device’s bandwidth. Testing with different balls indicates that the different sizes and material properties have different effects on the device’s performance, altering its mechanical and electrical damping.

Refrigerant leakage poses significant safety and environmental challenges in heating, ventilation, air conditioning, and refrigeration (HVAC&R) systems, particularly with the increasing use of highly flammable hydrocarbon (A3) refrigerants such as propane (R-290), ethane (R-170), butane (R-600), and isobutane (R-600a). Existing sensor technologies developed for traditional halogenated refrigerants are often unsuitable for accurately detecting low concentrations of hydrocarbons due to differences in chemical properties and flammability risks. This paper presents a comprehensive review of gas-sensing technologies applicable to A3 refrigerants, emphasizing both established and emerging technologies that could be adapted from other industries for use in HVAC&R applications. The sensor categories evaluated include metal–oxide semiconductor (MOS), catalytic, optical (photoacoustic spectroscopy—PAS, quartz-enhanced PAS, non-dispersive infrared—NDIR, fiber optic), acoustic (surface acoustic wave—SAW, quartz crystal microbalance—QCM), electrochemical, capacitive, and emerging nanomaterial-based sensors (C2N, sulfur-doped silicon carbide nanotube, surface plasmon resonance). Each technology was assessed based on critical parameters such as sensitivity, selectivity, response time, power consumption, and practicality for integration into HVAC&R systems. Although MOS, PAS/quartz-enhanced photoacoustic spectroscopy, and NDIR sensors demonstrate potential, limitations related to elevated operating temperatures, vibration sensitivity, and cross-selectivity remain significant concerns. Emerging technologies, including SAW, QCM, and novel nanostructured materials, exhibit promising performance characteristics such as room temperature operation, rapid response, high sensitivity, and compact size; however, they require further development and validation for reliability, long-term stability, and commercialization. This paper also identifies key gaps, challenges, and research opportunities, emphasizing the importance of developing robust calibration protocols and clearly defining operational conditions within HVAC&R systems to optimize sensor selection, safety, and system efficiency.

Electrostatic energy harvesters convert kinetic energy into electrical energy via variable capacitors. Efforts to improve their power output are hampered by a lack of understanding of the fundamental limit for energy conversion efficiency. In heat engines, the theoretical limit of conversion efficiency is intrinsically related to entropy and the second law of thermodynamics. Laying the foundation for similar concepts for kinetic energy harvesters may be necessary for establishing a conversion efficiency limit. Thus, the mixing entropy concept is borrowed from statistical mechanics and is adapted here, for the first time, to characterize the energy transfer between coupled mechanical–electrical oscillators. The investigated system is composed of a spring-mass coupled to an inductance-capacitor circuit via a variable capacitor. Combining the two subsystems (electrical and mechanical) generates entropy, referred to as mixing entropy. A non-dimensional study of the governing equations of the systems and their energy terms is carried out. Trends in mixing entropy are compared with trends in the total energy of the system, assuming a conservative system, weak coupling between electrical and mechanical domains, and identical natural frequency of the two oscillators. It is found that mixing entropy can predict the peak in effectiveness of the energy transfer between the two domains. For the cases studied, the maximum mixing entropy and effectiveness values occur when the ratio of the mechanical domain energy to the total energy of the system is 67%. The maximum effectiveness is independent of the initial conditions and depends on the squared ratio of the natural frequency of the nominal coupling capacitor to the natural frequency of the mechanical system.