Gravity affects the flow and the behavior at rest of complex fluids by inducing sedimentation, drainage, and interfacial deformation, which can mask more fundamental physical processes. On Earth, standard countermeasures against gravity come with limitations in terms of possible formulations or the quality and homogeneity of the applied strain. Alternatively, one can run these experiments in free-fall where the effects of gravity are canceled, i.e., under microgravity conditions. In this Short Review, we present several European Space Agency (ESA)-led projects of interest to rheologists that leverage microgravity environments, including interfacial rheology experiments using capillary pressure tensiometers; Fundamental experiments on the coarsening of foams and the intrinsic dynamics and microrheology of emulsion droplets; Multiple granular materials investigations on the transition to jamming, fluidized beds, quasistatic and dilute granular flows; Experiments examining the thermally driven perturbation of soft colloidal glasses; And studies of the aggregation dynamics and migration of soft particles and red blood cells under flow. The microgravity conditions offered by the ESA platforms enabled high-precision measurements of interfacial viscoelasticity; revealed plastic rearrangements in colloidal glasses; detected roaming bubbles in foams, and underlined the progressive arrest of droplet motion in emulsions; uncovered margination effects in blood cell analogues under flow; and rationalized the impact of gravity on convection and fluidization in agitated granular matter. Collectively, these experiments demonstrate the complementarity and the relevance of ESA’s microgravity platforms in expanding the frontiers of soft matter rheology.
Dripping-onto-substrate (DoS) rheometry is a well-established method for measuring the extensional rheology of low-viscosity liquids. However, clear guidelines on the capabilities and limitations of the technique are lacking. In the present work, we define operational limits for measuring a transient extensional viscosity directly from observations of the rate of filament thinning, as well as model-based bounds on calculating a viscosity η and extensional relaxation time τE of a liquid using DoS. Dilute solutions of polyethylene oxide and polyacrylamide are used to probe the lower limit of measurable τE, demonstrating that values as low as 0.1 ms can be resolved, provided (a) the intrinsic Deborah number (based on the ratio of the relaxation time and the Rayleigh breakup time scale) is De≥O(0.1) and (b) an instrumental constraint related to spatial and temporal resolution is satisfied. This instrumental constraint is quantified through a new metric we define as the filament capture rate, a “figure of merit” (expressed in s−1) that can be used to quantify the number of data points within the elastocapillary regime that are available for extraction of τE. Finally, model-agnostic bounds for reliable measurements of the transient extensional viscosity ηE+ using DoS rheometry are derived. These bounds prescribe experimental limitations due to inertia, gravity, and measurement resolution (spatial and temporal) that can assist rheologists in the design and interpretation of extensional viscosity measurements using DoS. Collectively, these results provide a quantitative roadmap for reliable DoS rheometry and affirm its use for measuring the transient extensional viscosity of liquid-like materials.
In this paper, a shear rheometer with a bottom quartz plate and a top reflective plate was modified to emit UV light [with wavelength ∼350–410 nm and absorbable light intensity ∼14.3 (W/m2)] on samples containing water, acrylic acid (0.15–0.30 gg−1), and photoinitiator (PI, 3.77–7.74 mM). The viscosity rise over time was used to in situ probe photoinduced free-radical polymerization (Photo-FRP). UV–vis measurements showed a small extinction coefficient [ε ∼29 1/(cm M)] at 357 nm with absorption coefficient ≤23 m−1, in line with the fact that sample thickness (0.55–2.2 mm) did not affect measurements, consistent with Beer–Lambert law predictions. Experimental steps involved oxygen removal, photon emission, inhibition, polymerization onset, and monitoring monomer conversion (X∼0%−99%). The inhibition period correlated with oxygen transport calculations. Size exclusion chromatography revealed Mw ∼2 000 000 g/mol and polydispersity ∼3.4. Frequency sweeps showed the progressive development of an elastic response during early polymerization (at ∼50 s), indicating polymer entanglement at Ce ≥ 0.005 gg−1 (∼0.33% monomer conversion). Viscosity–concentration relations (μ = 5.4*c0.99 for C < Ce and μ = 16.28*c4.28 for C > Ce) were used to estimate monomer conversion over time. A kinetic model assuming steady-state radical concentration and including inhibition, PI decomposition, propagation, and termination captured species evolution. Fractional free volume accounted for rate variations with monomer conversion, fitting experimental data well and yielding the photoinduced decomposition constant for the PI kd ∼ 1.75 × 10−5 1/s/(W/m2). The results of this work highlight the effectiveness of in situ rheology for studying liquid-state Photo-FRP and linking kinetic parameters to polymer molecular weight and process design.
Waxy oil gels exhibit complex rheological behavior, sharing characteristics with colloidal, polymeric, and fat gels. The phenomena of viscoplasticity, viscoelasticity, and thixotropy are manifested due to the dynamics of the fractal microstructure under cooling and shear conditions. Previous studies have developed models to probe these properties, including viscosity, storage modulus, yield stress, critical elastic strain and stress, and structure parameters. The models incorporate fractal characteristics within the theoretical framework. However, a unified approach to fractal rheological models for waxy oils is still lacking. This work aims to comprehensively review three types of fractal models for cluster suspensions and gels: viscosity models, elastoplastic models, and structure parameter models. The essential features of waxy oil rheology and the determination of yield stress are first outlined. Subsequently, models that describe the elastoplastic properties, such as storage modulus and yield stress, are described. Then, the viscosity models for fractal cluster suspensions are revisited, highlighting the occurrence of high correlation and multiple local minima in the regression procedure. Finally, the fractal-based structure parameters that connect the elastoplastic and viscosity models are examined. The review offers a comprehensive understanding of the complex rheological behavior of waxy oils, laying a foundation for future research in this field.
This paper is a Review Article on capillary rheometry for molten polymers as a versatile rheological tool. The capabilities of this technique to measure a number of fundamental rheological and other properties of molten polymers are reviewed. Specific examples are used to make the article illustrative, pedagogical, and effective to be used by rheologists working in the area of rheology and processing of molten polymers. These include (i) determination of flow curves to be compared with linear viscoelastic properties (Cox–Merz rule); (ii) effects of viscous heating on rheological measurements; (iii) study of wall slip of molten polymers; (iv) thermodynamic properties (density and isothermal compressibility); (v) the effects of pressure and temperature on the viscosity of molten polymers at high temperatures, pressures, and shear rates not accessible by conventional rotational rheometry; (vi) approximate methods to evaluate the extensional properties of polymers; (vii) exit pressure, hole pressure, and extrudate swell that are related to normal stress differences; (viii) processing properties and instabilities such as melt fracture and evaluation of effectiveness of processing aids to eliminate melt fracture; (ix) flow-induced crystallization under real processing conditions; and (x) mimicking polymer processing operations such as sheet extrusion or film casting (slit rheometry), profile extrusion (dies of any profile), tubing and wire coating extrusion (annular rheometry). The determination of this plethora of properties makes capillary rheometry an invaluable rheological tool that can complement rheological studies using rotational rheometry and, thus, bridge the gap between fundamental polymer rheology and polymer processing.
Molten polymers exhibit distinct slip characteristics over high-surface-energy metallic dies and low-surface-energy polymer processing aid (PPA) coatings. In this work, a methodology is developed to quantify the composite slip of molten polymers flowing over partially coated surfaces, where slip varies spatially between coated and bare regions. The proposed model is derived from a fundamental force balance governing pressure-driven flow and incorporates a recently formulated PPA coating dynamics framework describing the adsorption and desorption of PPA particles on the die wall [Jia et al., J. Rheol. 69, 495-505 (2025)]. The methodology is validated using capillary extrusion data for a linear low-density polyethylene melt, accurately capturing the evolution of wall shear stress and slip velocity associated with dynamic PPA coverage. The results demonstrate that the composite slip model provides a physically consistent description of the coupling between interfacial coating kinetics and macroscopic slip behavior in polymer flow.
Velocity-profiling-assisted rheometry (VPAR) based on particle image velocimetry (PIV) has been developed for the rheological characterization of complex multiphase fluids. In contrast to the conventional method based on ultrasonic velocity profiling, whose applicability is often hindered by attenuation or multiple reflections of ultrasonic waves in suspensions containing bubbles, droplets, or solid particles, the PIV-based VPAR offers high adaptability to complex multiphase fluids by measuring the surface flow between the concentric cylinders. As a demonstration, the effective viscosity of a bubble suspension in a xanthan gum-based non-Newtonian fluid was exemplarily investigated. A conventional torque-based interpretation misleadingly indicates that the bubbles decrease the effective viscosity at low shear rates, whereas at high shear rates they increase the effective viscosity relative to the single-phase condition. However, VPAR analysis revealed a consistent increase in the effective viscosity with bubble dispersion, aligning with established theoretical predictions. These findings highlight the risk of misinterpretation when using conventional rheometry for complex fluids and underscore the robustness of VPAR as a complementary methodology.
The progressive miniaturization of solid-state electronic systems drives device operation toward regimes characterized by increasingly rapid electronic dynamics, particularly in reduced-dimensional structures subject to quantum confinement. In this regime, the central issue is not only the intrinsic speed of electronic processes, but also the extent to which their temporal structure remains experimentally accessible through conventional readout chains. This work develops a phenomenological framework for the mismatch between intrinsic confined-state dynamics and the finite temporal resolution of the measurement front-end. The physical ingredients involved—quantum confinement, finite bandwidth, and detector-mediated filtering—are individually well established; the contribution here is to combine them into an explicit observability-based description with operational regime descriptors. To this end, we introduce a dimensionless observability parameter that identifies the crossover between observable and observability-limited regimes, together with an observability transfer efficiency that quantifies the fraction of intrinsic dynamical content preserved by the readout chain. A minimal analytical model shows that the same control parameter governs retained spectral content, signal attenuation, and peak delay, thereby linking spectral filtering to measurable time-domain distortions. Within this formulation, the measurable electrical response is treated as a property of the coupled system formed by the electronic states and the front-end, rather than of the confined system in isolation. The resulting framework provides a compact basis for analyzing observability limits in nanometric electronic systems and for identifying experimentally testable crossover behavior when intrinsic and instrumental timescales become comparable.
The competition between incompatible oxygen octahedral tilting modes across heterointerfaces offers a powerful, yet underexplored, avenue to discover emergent phenomena. Here, we report an atomically switchable giant spontaneous Hall effect (SHE) in nTbScO3/10SrIrO3 superlattices. By controlling the TbScO3 spacer thickness with a single-unit-cell precision, we observed an abrupt increase of SHE at n = 1, above which SHE is significantly suppressed to a Berry curvature dominated level. Synchrotron x-ray diffraction and scanning transmission electron microscopy reveal that this switching behavior arises from a nontrivial modulation of octahedral tilting in the superlattice unit, leading to an incipient modulation in magnetic structure.
While two-dimensional silicon carbide (SiC) shows promise for nanodevices, its optoelectronic applications are limited by its intrinsic indirect bandgap and strong electron-phonon scattering. To overcome these bottlenecks, the construction of GeC/SiC van der Waals heterostructures is proposed herein as an effective solution. Based on first-principles calculations and the GW-BSE (GW-Bethe-Salpeter equation) formalism, the underlying physical mechanisms by which this architecture resolves the aforementioned limitations are systematically elucidated. First, the isostructural coupling between GeC and SiC, combined with stacking engineering, is shown to induce a distinct type-II band alignment that facilitates spontaneous charge separation and robust electric-field tunability. Subsequently, addressing the transport limitations, acoustic deformation potential analysis indicates that the unique orbital hybridization delocalizes conduction band states, effectively reducing the electron effective mass, while simultaneously increasing the in-plane lattice stiffness by a factor of two. This synergistic interaction significantly suppresses intrinsic acoustic phonon scattering, resulting in an electron mobility nearly sevenfold higher than that of isolated SiC. Finally, mitigating the bandgap constraints, spatially indirect excitons are found to dynamically screen the Coulomb interaction, driving a redshift of the optical absorption edge from the ultraviolet to the near-infrared region. These findings illuminate the interfacial charge and transport dynamics and provide a theoretical foundation for designing high-performance two-dimensional optoelectronic devices.
The driving and distribution of fluid is important for building integrated and functional microfluidic systems. This study presents a bidirectional micropump based on acoustically oscillating water droplet-shaped microbubbles. The micropump exploits the geometric asymmetry of the microbubbles along the pumping direction to generate asymmetric acoustic microstreaming actuated with acoustic waves, enabling directional fluid transport. Bidirectional pumping is achieved by adjusting the frequency of actuated acoustic waves, which switches the direction of the acoustic microstreaming. The prototypes were fabricated, and experimental tests were conducted. Experimental results show that water droplet-shaped microbubbles generate out-of-plane microstreaming near the base ends when the frequency of the actuated acoustic waves is between 11.85 and 13.11 kHz. As a result of that, the net flow from the base end to the tip of the water droplet-shaped microbubbles is formed by the superposition of the array of acoustic microstreaming. The micropump achieved its peak forward flow of 2658 nl/min at an acoustic frequency of 12.32 kHz and 60 Vpp. A reverse out-of-plane microstreaming is generated at the middle of the microbubbles when the frequency of acoustic waves is within the range of 18.22-19.35 kHz. A reverse flow from the tip to the base end is generated by the superposition of acoustic microstreaming produced by the microbubble array. A peak reverse flow of 1230 nl/min was achieved at 18.55 kHz and 140 Vpp. This study proposes a flexible bidirectional micropump, offering a solution for the dynamic regulation of flow direction in microfluidic systems.
Applications such as observing transient phenomena in materials and real-time terahertz (THz) spectroscopic imaging of molecules have highlighted the importance of high repetition rate, high power THz radiation sources with a broad frequency tuning range. Nevertheless, the development of such sources remains a bottleneck, hindering the advancement of these applications. We have recently developed a high repetition rate, picosecond-pulsed THz parametric amplifier system driven by a passively mode-locked femtosecond fiber laser, which can be synchronized to an external reference signal with sub-picosecond timing jitter. The system operates stably with a frequency tuning range from 1 to 6 THz. It achieves a maximum peak power of 4.3 kW at 5.7 THz when operating at 10 kHz and a maximum average power of 470 mu W at the same frequency under 100 kHz operation. This THz source can not only serve as a seed for THz free-electron laser amplifiers, but also provide precise timing control for various THz applications.
Here, a metasurface design strategy with high-quality factors and figure of merit (FOM) value is proposed and verified in the 4.0-20.0 THz range. By introducing symmetry breaking, the disk array is transformed into a tetramer cluster structure. The electromagnetic dipole-hybrid (ED-MD) mode and the magnetic toroidal dipole (MTD) mode are excited by the proposed metasurface. The ED-MD mode and MTD mode show high quality factors and extremely narrow bandwidth, which are beneficial for clothing fabric sensing applications. The amplitude and resonance positions can be modulated by using asymmetric parameters. The ED-MD mode exhibits a measured refractive index sensitivity of 921.25 GHz/RIU, and the corresponding FOM value is 3.56 & times; 10(4) in the clothing fabric sensing. This proposed metasurface verifies the asymmetric resonance characteristics and the clothing fabric sensing applications.
The transverse structure of field-emission electron wave packets is governed by the cathode material and geometry through the near-apex electrostatic potential, which induces electrostatic defocusing and spatial transmission filtering. To study these effects, we develop a framework for field-emission wave-packet propagation that combines finite-element electrostatic potentials with paraxial Schr & ouml;dinger propagation under an effective parabolic transverse-potential approximation, together with barrier-side spatial filtering. Applied to nanometer-scale prolate hemispheroidal emitters, the method enables efficient wave-packet calculations that reproduce the expected electron-optical defocusing and show how tunneling-induced narrowing can amplify subsequent transverse expansion. We further apply the framework to a two-dimensional film-edge model of a graphene edge emitter, where the & Aring;-scale initial width leads to stronger transverse expansion. The resulting wave packets are then used to study coherent multi-site superposition, predicting either overall beam narrowing or fragmentation into multiple beamlets, which may contribute to the striated field-emission-microscopy patterns reported for graphene edges.
The accumulation of transmutant helium (He) poses a major challenge to the structural integrity of materials in the nuclear industry. To elucidate the He effects on microstructural evolution, we performed molecular dynamics simulations of overlapping collision cascades in copper containing pre-existing substitutional He, at concentrations up to 10 000 appm and a cumulative dose of similar to 0.24 dpa. Based on statistical analysis across multiple independent simulation runs, the results reveal a synergistic evolution between He atoms and radiation-induced defects. In contrast to the large, localized dislocation loops formed in pristine Cu, He-containing samples developed a distinct damage structure characterized by smaller loops and homogeneously distributed defect clusters. This morphology originates from dominant continuous recombination-replacement reactions between self-interstitials and substitutional He, which eject substantial amounts of He into interstitial sites. These He atoms constitute a significant fraction of the interstitial clusters, and their formation into Cu-He complexes severely restricts cluster mobility. Consequently, the agglomeration and growth of dislocation loops are suppressed. Simultaneously, an interstitial-mediated mechanism drives pronounced He segregation and initial bubble nucleation. The atomic-scale insights provided by this study are crucial for understanding He-induced microstructural degradation under prolonged irradiation.
Industrial dyes such as methylene blue (MB) and methyl orange (MO) pose significant ecological risks when released into aquatic systems. In this study, titanium dioxide nanoparticles (TiO2 NPs) were synthesized via a green synthesis approach using Azadirachta indica flower extract and titanium tetraisopropoxide. The bioactive compounds in the extract acted as reducing and stabilizing agents, promoting the formation of anatase-phase TiO2. The synthesized nanoparticles were characterized using UV-visible spectroscopy, X-ray diffraction (XRD), Fourier transform infrared spectroscopy, and transmission electron microscopy (TEM). UV-vis analysis confirmed a bandgap energy of 3.2 eV, while XRD results indicated a tetragonal anatase structure. FT-IR spectra showed reduced intensities of N-H and C=O functional groups, confirming the role of biomolecules in nanoparticle stabilization. TEM images revealed predominantly spherical nanoparticles with sizes below 20 nm. Photocatalytic degradation studies demonstrated high efficiency, with maximum degradation of 99.30% for MB at pH 8 and 99.17% for MO at pH 5. The degradation followed first-order kinetics, with rate constants of 0.0403 min(-1) for MB and 0.00773 min(-1) for MO. These findings highlight the potential of green-synthesized TiO2 nanoparticles as an effective and sustainable photocatalyst for wastewater treatment.
We report the successful integration of a bilayer ferroelectric aluminum-scandium-nitride (Al0.64Sc0.36N) thin film on an industrial-scale 200 mm silicon (100) wafer. Microstructural analysis revealed that both the bottom and top Al0.64Sc0.36N layers exhibit comparable characteristics. Wafer-scale ferroelectric and piezoelectric characterization of the individual layers showed similar average values for permittivity (epsilon), loss tangent (tan delta), and piezoelectric coefficients (d(33,f) and e(31,f)). Findings suggest that the bottom Al0.64Sc0.36N layer has a lower leakage current than the top layer in the bilayer configuration. We present a strategy for efficiently actuating both the top and bottom layers to generate a high equivalent longitudinal strain and transverse stress without requiring any complex or additional electrical circuitry. Upon excitation, the bilayer structure yields equivalent d(33,f) and e(31,f) values of >26.8 pm/V and >4.6 C/m(2), respectively, among the highest ever reported within bilayers of complementary metal-oxide-semiconductor (CMOS) compatible ferroelectric nitride thin films. Such high piezoelectric coefficients enable the broader adoption of lead-free ferroelectric materials for actuator applications.
We present a systematic first-principles study of dopant-induced phase stabilization in HfO2 across ferroelectric (FE) and non-ferroelectric polymorphs for a large set of dopants. To overcome the strong configuration dependence of defect energetics, we develop a multi-stage screening workflow that identifies low-energy dopant-vacancy configurations. In an equilibrated-solution ["PVD" (physical vapor deposition)] model, bulk T = 0 K energetics indicate that ionically compensated doping alone is insufficient to stabilize the orthorhombic-ferroelectric phase over the monoclinic phase; certain dopants even increase its relative energy. Configurational-entropy corrections are small at device-relevant temperatures, and vibrational contributions-benchmarked against explicit phonon calculations-primarily stabilize the tetragonal phase, remaining inadequate to reverse bulk phase ordering at moderate anneal temperatures. We further observe that many rhombohedral-FE supercells lose phase identity upon relaxation, typically collapsing toward orthorhombic-FE motifs, underscoring sensitivity to local defect arrangements. To assess deposition effects, we introduce a planar ("atomic layer deposition") model that mimics dopant layering; it amplifies dopant-identity sensitivity and can reshape phase competition compared to the equilibrated-solution limit. Overall, our results suggest that experimentally observed ferroelectric stabilization in thin films arises from a combination of interfacial/finite-size terms and deposition-induced dopant distributions, rather than bulk thermodynamics alone.
We report a programmable multiple ion beam system developed by combing a magneto-optical trap ion source with a spatial light modulator (SLM). Neutral rubidium atoms are trapped into a magneto-optical trap and cooled to 700 mu K. An excitation laser beam, the intensity distribution of which is shaped into a square array by the SLM, selectively excites the cold atoms, which are subsequently ionized by an ionization laser beam. The ions are then extracted by an electric field to produce multiple ion beams. A 3 & times;3 ion beam array has been generated. The size of each beams is 180 mu m, and the transversal reduced emittance of multiple ion beams at the source is evaluated to be 1.65 & times; 10(-9) m rad root eV. We also demonstrate the generation of ion bunches with a user-defined spatial pattern. In the future, by integrating this programmable multiple ion beam system with high reduction ion optics, it is possible to generate nanoscale multiple focused ion beams for the potential applications in maskless patterning and direct writing lithography.
