Max Planck University of Twente Center for Complex Fluid Dynamics

Accurate sizing of ultrasound contrast agent microbubbles is crucial for understanding their behavior through a direct comparison between models and experimental data. However, a gold standard method for microbubble sizing has yet to be established. We introduced optical attenuation spectroscopy (OAS) for the sizing of monodisperse microbubble suspensions. This method employs a spectrophotometer to obtain an optical attenuation spectrum, to which a Mie scattering model is fitted, yielding the modal bubble radius and polydispersity index. We compared OAS sizing with the most common methods: Coulter Counter, brightfield microscopy and fluorescence microscopy. OAS measurements were in excellent agreement (±1.5%) with fluorescence microscopy. In contrast, the Coulter Counter over-estimated the modal microbubble radius compared with OAS by 4% for 3.0 μm radius microbubbles, increasing up to 11% for 4.0 μm radius microbubbles. Brightfield microscopy systematically over-estimated the microbubble radius by 0.45 μm compared with fluorescence microscopy. OAS has proven to be a versatile method for sizing monodisperse microbubble suspensions due to its convenient, label-free, and rapid acquisition within seconds.

Microdroplets have emerged as powerful and sustainable platforms for the design and synthesis of functional materials under mild and environmentally friendly conditions. Their unique physicochemical environments ‐ characterized by high surface‐to‐volume ratios and confined internal space ‐ enable precise control over mass and heat transfer, interfacial energy conversion, and chemical reactions. These features have been harnessed in two main ways: first, by employing microdroplets as microreactors for the fabrication of advanced materials such as polymeric microlenses, artificial compound eyes, metal oxide nanoparticles, and metal‐organic framework microstructures; and second, by using microdroplets as reactive entities to accelerate interfacial reactions relevant to hydrogen and biodiesel production, as well as nitrogen and carbon dioxide fixation. Together, these strategies have driven significant advances in clean energy generation, environmental monitoring, and water treatment. This review provides a critical overview of recent progress in microdroplet‐assisted synthesis of functional materials and their integration in energy and environmental technologies. An emerging direction in the integration of microdroplet‐based systems into adaptive sensing and human‐machine interfaces driven by artificial intelligence is also highlighted.

The aspect ratio effect on side and basal melting in fresh water is systematically investigated across a range of Rayleigh numbers and ambient temperatures using direct numerical simulations. The side mean melt rate follows a ${Ra}^{1/4}\,\gamma ^{-3/8}$ scaling relation in the side-melting dominant regime, where ${Ra}$ is the Rayleigh number, and $\gamma$ is the width-to-height aspect ratio of the ice block. In the basal-melting dominant regime, the basal mean melt rate follows a ${Ra}^{1/4}\gamma ^{3/8}$ scaling relation at low Rayleigh numbers, but transitions to a ${Ra}^{1/3}\gamma ^{1/2}$ scaling relation at higher Rayleigh numbers. This scaling transition is attributed to the formation of a bottom cavity resulting from flow separation at high Rayleigh numbers. The overall mean melt rate exhibits a non-monotonic dependence on the aspect ratio, driven by the competition between side and basal melting. The proposed theoretical model successfully captures the observed non-monotonic behaviour, and accurately predicts the overall mean melt rate over the considered range of Rayleigh numbers and ambient temperatures, especially in the side- and basal-melting dominant regimes. More specifically, the side, basal and overall mean melt rates follow a linear ${St}$ scaling relation for ambient temperatures $T_{w}\geqslant 15^{\,\circ }\textrm {C}$ , with ${St}$ being the Stefan number (the ratio between sensible heat and latent heat), but deviations from this scaling relation and a non-monotonic dependence on the ambient temperature are observed at lower ambient temperatures, which can be attributed to the density anomaly effect.

Bubble bursting and subsequent collapse of the open cavity at free surfaces of contaminated liquids can generate aerosol droplets, facilitating pathogen transport. After film rupture, capillary waves focus at the cavity base, potentially generating fast Worthington jets that are responsible for ejecting the droplets away from the source. While extensively studied for Newtonian fluids, the influence of non-Newtonian rheology on this process remains poorly understood. Here, we employ direct numerical simulations to investigate the bubble cavity collapse in viscoelastic media, such as polymeric liquids. We find that the jet and drop formations are dictated by two dimensionless parameters: the elastocapillary number $Ec$ (the ratio of the elastic modulus and the Laplace pressure) and the Deborah number $De$ (the ratio of the relaxation time and the inertio-capillary time scale). We show that, for low values of $Ec$ and $De$ , the viscoelastic liquid adopts a Newtonian-like behaviour, where the dynamics is governed by the solvent Ohnesorge number $Oh_s$ (the ratio of visco-capillary and inertio-capillary time scales). In contrast, for large values $Ec$ and $De$ , the enhanced elastic stresses completely suppress the formation of the jet. For some cases with intermediate values of $Ec$ and $De$ , smaller droplets are produced compared with Newtonian fluids, potentially enhancing aerosol dispersal. By mapping the phase space spanned by $Ec$ , $De$ and $Oh_s$ , we reveal three distinct flow regimes: (i) jets forming droplets, (ii) jets without droplet formation and (iii) absence of jet formation. Our results elucidate the mechanisms underlying aerosol suppression versus fine spray formation in polymeric liquids, with implications for pathogen transmission and industrial processes involving viscoelastic fluids.

We study the dynamics of a thin liquid sheet that flows upwards along the sides of a vertically aligned, impacting plate. Upon impact of the vertical solid plate onto a liquid pool, the liquid film is ejected and subsequently continues to flow over the solid surface while the plate enters the water. With increasing impact velocity, the liquid film is observed to rise up faster and higher. We focus on the time evolution of the liquid film height and the thickness of its upper rim and discuss their dynamics in detail. Similar to findings in previous literature on sheet fragmentation during drop impact, we find the rim thickness to be governed by the local instantaneous capillary number based on gravity and the deceleration of the liquid sheet, showing that the retraction of the rim is primarily due to capillarity. In contrast, for the liquid film height, we demonstrate that the viscous dissipation in the thin boundary layer is the dominant factor for the vertical deceleration of the liquid sheet, by modelling the time evolution of the film height and showing that the influences of capillarity, gravity and deceleration due to the air phase are all negligible compared with the viscous term. Finally, we introduce characteristic viscous time and length scales based on the initial rim thickness and show that the maximum height of the film and the corresponding time can be determined from these viscous scales.

This work evaluates the potential impact of interface traps on the interface of oxide and semiconductor on a 2-D-based ion-sensitive field-effect transistor (ISFET) using a surface potential-based model and Technology Computer-Aided Design (TCAD) simulation calibrated for pH sensing applications. The electrolyte/oxide interface in the proposed 2-D ISFET model is modeled using the Guoy-Chapman–Stern technique and the site binding association model. The baseline field-effect transistor (FET) of the ISFET is modeled using Fermi-Dirac statistics to obtain surface potential, which is further used to derive a compact drain current expression. The proposed ISFET model development is carried out by accounting for the interface trap effect. The voltage and current sensitivity of ISFET with and without interface traps are calculated to demonstrate the impact of interface traps in 2-D material-based pH sensors. It is observed that voltage sensitivity remains close to the Nernst limit (59 mV/pH), and a decrease in current sensitivity from 34 to 2 nA/pH is observed, as the density of interface traps is varied from 0 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${5}\times {10} {^{{12}}}$ </tex-math></inline-formula>cm<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-}2 $ </tex-math></inline-formula>eV<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-}1 $ </tex-math></inline-formula>. Another sensitivity metric, the transconductance to drain current ratio, is analyzed for its variation with changes in the density of interface traps. The noise level of ISFET in the presence of interface traps is analyzed, and its influence on the minimum resolvable pH is demonstrated. The proposed model prediction closely matches the TCAD simulation data obtained from a calibrated TCAD simulation setup. The model is suitable for implementation in Verilog-A for ISFET-based circuit simulation, and the impact of interface traps on the response time of ISFET-based circuits is also demonstrated.

Rapid detection of uremic toxins is crucial due to their severe health risks, including oxidative stress, inflammation, neurotoxicity, cardiovascular complications, and progression of chronic kidney disease. Surface-enhanced Raman spectroscopy (SERS) may provide sensitive, fast, and clinical-grade real-time monitoring of these toxins, enabling effective management with timely dialysis treatments. This study introduces a 3D-printed microchamber that integrates the fabrication of plasmonic metal nanoparticles for the in-flow detection of biological toxins and pharmaceutical drugs using SERS, making it ideal for on-site diagnostics in clinical settings. The microchamber supports quantitative and highly reproducible detection with liquid volumes under 100 μL, which is crucial for trace-level biomarker detection and minimizing cross-contamination. It employs a tunable solvent exchange method for the in situ synthesis of silver nanoparticles (AgNPs) on flexible PDMS or rigid Si wafer substrates, avoiding costly nanofabrication techniques. Ultralow detection limits were achieved for two model compounds and three pharmaceutical drugs: 10-11 M for rhodamine 6G, 10-7 M for adenine, and 10-6 M for the pharmaceutical drugs. A total of 13 biological toxins, including three neurotransmitters, one neuromodulator, five amino acids, two polyamines, and two urea cycle metabolites, were detected with quantitative limits ranging from 10-3 to 10-6 M, all below permissible levels and aligning with physiological conditions. SERS detection within microchambers facilitates rapid on-site analysis, proving ideal for personalized health monitoring, point-of-care diagnostics, and environmental pollution assessment.

The performance of wind farms strongly depends on the prevailing atmospheric conditions. We investigated how baroclinicity, caused by horizontal temperature gradients in the atmosphere, affects wind farm performance and wake recovery, using large eddy simulations. Baroclinicity impacts the power generation in the entrance region of a wind farm by modifying atmospheric conditions around turbine height, such as the turbulence intensity. The power production of downstream turbines is also affected by baroclinicity, as it alters the kinetic energy available for entrainment above the wind farm. Furthermore, our findings reveal that the recovery of wind farm wakes is governed by wake expansion, controlled by atmospheric turbulence intensity, and by an upward shift of the wake velocity deficit, which is driven by vertical velocity shear. These insights have been incorporated into a novel engineering model designed to predict large-scale wake recovery behind wind farms in both barotropic and baroclinic conditions.

This study demonstrates the use of degenerate primers in combination with CRISPR-based detection for high selectivity measurements of high-risk HPV types. We found that the E6 genes are more suitable for this purpose than the L1 gene, and the lbCas12a performs better than AsCas12a in differentiating between HPV types. Initial steps toward integrating this method into an easy-to-use device have been taken, with future research focusing on incorporating the entire process, including DNA extraction, target amplification, and target detection, into a single device. This advancement aims to make primary HPV screening feasible in low-resource settings, improving accessibility and early detection capabilities.

Dynamically stable traveling periodic waves are notoriously difficult to realize experimentally. Manifestations of such waves inherently become unstable in any dispersive medium, whether it be at continuum length scales in fluid mechanics, in electrodynamics, or in nonlinear optics. Here, a simple experimental system is proposed where Stokes waves are emitted from a resonator whose cavity accommodates standing waves of same wavelength that is emitted out. A single characteristic wavelength is found for each driving frequency despite the coexistence of standing and traveling waves, external noise, and potentially parasitic reflections. The system's response is shown to be reliable at very small values of quality factor. A unique utility of this model system is suggested in that such a venting resonator can be used as an inhibitor of modulation instability in dispersive medium where an engineering application demands stable periodic traveling waves. Some parallels are drawn with other unbounded resonators found in society, whereby their leakiness is central to their application, such as the role of air-filled cavity to achieve impedance matching in stringed musical instruments, or energy harvesting from oscillating fluid columns in nature.