When a highly elastic drop of a polymer solution hits a superhydrophobic surface at a high speed, a growing tail-like filament emerges vertically from the impact spot as the contact line recedes. Notably, the ligament transitions into a balloon-like shape before detaching completely from the surface (Balloon regime). The ligament formation is attributed to liquid impalement upon impact into the surface protrusion spacing, and elastic forces due to polymers prevent ligament breakup. The detachment of the ligament happens when polymeric stresses balance or overcome the adhesion at the surface. This study shows that tuning droplet rheology and surface roughess enables droplets to rebound completely and without splashing at high impact speeds. 

The mechanical response of yield-stress materials below the yield point remains a subject of debate. Two of the most widely used constitutive models for these materials offer fundamentally conflicting views: one permits plastic flow at all stress levels, while the other assumes entirely recoverable viscoelasticity below yield. Using parallel superposition rheometry, we test the subyield behavior of a microgel and an emulsion. When residual slip effects are properly accounted for, both fluids exhibit bounded, periodic strain responses, offering compelling evidence that they do not flow in the studied regime. Our results indicate that the subyield regime is underpinned by nonlinear viscoelasticity and underscore the need for improved constitutive relations that capture such effects without treating yielding as a precursor for nonlinearity.

Multiphase flows where particles, bubbles, or droplets are suspended in a fluid govern critical processes in biology, medicine, materials processing, and geophysics. However, observing their microscale dynamics in opaque systems has remained a fundamental challenge. We present Synchrotron X-ray Multi-Projection Imaging (XMPI), a novel approach enabling four-dimensional (3D + time) tracking of microparticles in visibly opaque suspension flows without requiring sample rotation. By capturing simultaneous projections from multiple angles using beam-split X-rays at synchrotron facilities, we resolve instantaneous particle positions and trajectories in opaque fluids such as blood. We demonstrate the potential of XMPI through individual particle tracking velocimetry (3D PTV) in dilute conditions, as well as multi-projection image velocimetry in dense suspensions. The methodology provides otherwise inaccessible experimental validation for particle-resolved computational fluid dynamics models and allows, e.g., observation of inertial focusing effects and microstructural dynamics relevant to suspension rheology and biomedical flows. This work paves the way for high-resolution, time-resolved 4D imaging of complex multiphase flows across a range of scientific and industrial applications. Combining XMPI with recent AI-supported 4D reconstruction algorithms opens a new spatiotemporal frontier for high-speed, rotation-free microtomography.

Elasto-inertial microfluidics enables precise particle focusing and separation, but most experimental studies rely on fluorescence microscopy, which provides only 2D information and cannot resolve out-of-plane motion. Here, we combine fluorescence imaging with Optical Coherence Tomography (OCT) to obtain complementary lateral and depth-resolved (resolution along the beam: 2.58 μm in the present medium) particle distributions in microfluidic channels. Using 3 μm and 5 μm particles in a PEO solution at flow rates of 1–50 μL/min, fluorescence microscope captures lateral focusing followed by lateral defocusing with the increasing flow rate, while OCT reveals the vertical distribution of particles at the focusing and defocusing states. These results demonstrate that OCT is a promising method to obtain essential 3D information in elasto-inertial flows that complements fluorescence microscopy and enables a more complete understanding of particle behavior in elasto-inertial flows.

We here explore confinement-induced assembly of whey protein nanofibrils (PNFs) into microscale fibers using micro-focused synchrotron X-ray scattering. Solvent evaporation aligns the PNFs into anisotropic fibers and the process is followed in situ by scattering experiments in a droplet of PNF dispersion. We find an optimal temperature at which the order of the protein fiber has a maximum, suggesting that the degree of order results from a balance between the time scales of the forced alignment and the rotational diffusion of the fibrils. Moreover, we observe that the assembly process depends on the nano-scale morphology of the PNFs. Stiff PNFs with a persistence length in the micrometer scale are aligned at the air-water interface and the anisotropy gradually decrease towards the center of the droplet. Marangoni flows often increase entanglements toward the center, leading to complex patterns in the droplet. Flexible fibrils with a short persistence length (< 100 nm) tends to align uniformly throughout the droplet, possibly due to stronger local entanglements. Straight PNFs form smaller clusters with shorter inter-cluster distances due to their tight packing and consistent linear structure. In contrast, curved PNFs form intricate networks with larger characteristic distances and more varied structures because of their flexibility and adaptability.

Robust surfaces capable of reducing flow drag, controlling heat and mass transfer, and resisting fouling in fluid flows are important for various applications. In this context, textured surfaces impregnated with a liquid lubricant show promise due to their ability to sustain a liquid–liquid interface that induces slippage. However, theoretical and numerical studies suggest that the slippage can be compromised by surfactants in the overlying fluid, which contaminate the liquid–liquid interface and generate Marangoni stresses. In this study, we use Doppler-optical coherence tomography, an interferometric imaging technique, combined with numerical simulations to investigate how surfactants influence the slip length of lubricant-infused surfaces with longitudinal grooves in a laminar flow. Surfactants are endogenously present in the contrast agent (milk) which is added to the working fluid (water). Local measurements of slip length at the liquid–liquid interface are significantly smaller than theoretical predictions for clean interfaces (Schönecker & Hardt 2013). In contrast, measurements are in good agreement with numerical simulations of fully immobilized interfaces, indicating that milk surfactants adsorbed at the interface are responsible for the reduction in slippage. This work provides the first experimental evidence that liquid–liquid interfaces within textured surfaces can become immobilised in the presence of surfactants and flow.

Optical coherence tomography (OCT) has become an indispensable tool for investigating mesoscopic features in soft matter and fluid mechanics. Its ability to provide high-resolution, non-invasive measurements in both spatial and temporal domains bridges critical gaps in experimental instrumentation, enabling the study of complex, confined, and dynamic systems. This review serves as both an introduction to OCT and a practical guide for researchers seeking to adopt this technology. A set of tutorials, complemented by Python scripts, is provided for both intensity- and Doppler-based techniques. The versatility of OCT is illustrated through case studies, including time-resolved velocimetry, particle-based velocity measurements, slip velocity characterization, detection of shear-induced structures, and analysis of fluid-fluid and fluid-structure interactions. Drawing on our experiences, we also present a set of practical guidelines for avoiding common pitfalls.

Properties and functions of materials assembled from nanofibrils critically depend on alignment. A material with aligned nanofibrils is typically stiffer compared with a material with a less anisotropic orientation distribution. In this work, we investigate nanofibril alignment during flow focusing, a flow case used for spinning of filaments from nanofibril dispersions. In particular, we combine experimental measurements and simulations of the flow and fibril alignment to demonstrate how a numerical model can be used to investigate how the flow geometry affects and can be used to tailor the nanofibril alignment and filament shape. The confluence angle between sheath flow and core flow, the aspect ratio of the channel and the contractions in the sheath and/or core flow channels are varied. Successful spinning of stiff filaments requires: (i) detachment of the core flow from the top and bottom channel walls and (ii) a high and homogeneous fibril alignment. Somewhat expected, the results show that the confluence angle has a relatively small effect on alignment compared with contractions. Contractions in the sheath flow channels are seen to be beneficial for detachment, and contractions in the core flow channel are found to be an efficient way to increase and homogenise the degree of alignment.

The complex architecture of wood motivates studies of bioinspired materials that combine strength, toughness, and mechanical integrity. We explore the interplay between nanofiber alignment and molecular interactions in composite filaments formed from cellulose nanofibers (CNFs) and a dendritic polyampholyte, Helux. Helux enhances strength by 60% and increases toughness 5-fold through ionic bonding and thermal covalent cross-linking. However, wide-angle X-ray scattering (WAXS) reveals reduced nanofiber alignment in Helux-containing samples, resulting in a 25% decrease in stiffness-highlighting a trade-off between structural order and cohesion. Polarized optical microscopy (POM) and in situ small-angle X-ray scattering (SAXS) attribute this reduced alignment to enhanced rotary diffusion, driven by carboxylate groups of the Helux. With Helux, multivalent links across the nanofibers give a denser and tougher network with fewer voids. This behavior resembles lignin and hemicellulose interactions in wood, where flexibility and cohesion govern the performance.

Lubricated textured surfaces immersed in liquid flows offer tremendous potential for reducing fluid drag, enhancing heat and mass transfer, and preventing fouling. According to current design rules, the lubricant must chemically match the surface to remain robustly trapped within the texture. However, achieving such chemical compatibility poses a significant challenge for large-scale flow systems, as it demands advanced surface treatments or severely limits the range of viable lubricants. In addition, chemically tuned surfaces often degrade over time in harsh environments. Here, we demonstrate that a lubricant-infused surface (LIS) can resist drainage in the presence of external shear flow without requiring chemical compatibility. Surfaces featuring longitudinal grooves can retain up to 50% of partially wetting lubricants in fully developed turbulent flows. The retention relies on contact-angle hysteresis, where triple-phase contact lines are pinned to substrate heterogeneities, creating capillary resistance that prevents lubricant depletion. We develop an analytical model to predict the maximum length of pinned lubricant droplets in microgrooves. This model, validated through a combination of experiments and numerical simulations, can be used to design chemistry-free LISs for applications where the external environment is continuously flowing. Our findings open up new possibilities for using functional surfaces to control transport processes in large systems.