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We propose a phase-field model to study interfacial flows of nematic liquid crystals that couple the capillary forces on the interface with the elastic stresses in the nematic phase. The theoretical model has two key ingredients: A tensor order parameter that provides a consistent description of the molecular and distortional elasticity, and a phase-field formalism that accurately represents the interfacial tension and the nematic anchoring stress by approximating a sharp-interface limit. Using this model, we carry out finite-element simulations of drop retraction in a surrounding fluid, with either component being nematic. The results are summarized by eight representative steady-state solutions in planar and axisymmetric geometries, each featuring a distinct configuration for the drop and the defects. The dynamics is dominated by the competition between the interfacial tension and the distortional elasticity in the nematic phase, mediated by the anchoring condition on the drop surface. As consequences of this competition, the steady-state drop deformation and the clearance between the defects and the drop surface both depend linearly on the elastocapillary number.

Because of their unique electronic structure and peculiar physical properties, topological materials have attracted considerable attention in condensed matter physics. In both physics and material science, exploring materials with a topological state or Dirac fermions near the Fermi level is important. Here, we report the transport evidence of the topological state for ${\mathrm{Ag}}_3\mathrm{Sn}$. Under a high magnetic field, we observed clear Shubnikov–de Haas (SdH) oscillations with three major frequencies and relatively small effective masses. Our SdH quantum oscillation analyses reveal that the Berry phase of the $\gamma$ band is very close to the nontrivial value $\pi$; the nonzero Berry phase for the $\alpha$ band indicates the ${\mathrm{Ag}}_3\mathrm{Sn}$ alloy to be a possible topological material. Moreover, we performed angle-resolved magnetoresistance (MR) measurements. Note that MR displays an unusual angular dependence, which can be tuned from twofold to fourfold symmetry by temperature and field. This feature makes ${\mathrm{Ag}}_3\mathrm{Sn}$ a candidate for valleytronics.

Tuning superconductivity in topological materials by means of chemical substitution, electrostatic gating, or pressure is thought to be an effective route towards realizing topological superconductivity with their inherent Majorana fermions, the manipulation of which may form the basis for future topological quantum computing. It has recently been established that the pseudo-binary chalcogenides (${ACh)}_m{\left(P{n}_{2}C{h}_3\right)}_n$ ($A=\text{Ge}$, Mn, Pb, etc.; $Pn=\text{Sb}$ or Bi; $Ch=\text{Te}$, Se) may host novel topological quantum states such as the quantum anomalous Hall effect and topological axion states. Here we map out the phase diagram of one member in this series, the topological insulator candidate ${\mathrm{GeSb}}_4{\mathrm{Te}}_7$ up to pressures of $\sim 35$ GPa, through a combination of electrical resistance measurements, Raman spectroscopy, as well as first-principles calculations. Three distinct superconducting phases emerge under the pressure above $\sim 11, \sim 17$, and $\sim 31$ GPa, which are accompanied by concomitant structural transitions, evidenced from the changes in the Raman modes. The first-principles calculations validate the existence of a topological insulating state at ambient pressure and predict two possible structural transitions at 10 and 17 GPa, in agreement with the experimental observations. Overall, our results establish the ${\mathrm{GeSb}}_4{\mathrm{Te}}_7$ family of materials as a fertile arena for further exploring various topological phenomena, including topological phase transitions and putative topological superconductivity.

We report experiments on lateral migration of bubbles in a two-dimensional foam sheared in a narrow-gap Couette device. A larger bubble in an otherwise monodisperse bubble raft migrates toward the center of the gap as long as the bubble size ratio and the shear rate are each above a threshold. The migration speed is roughly two orders of magnitude higher than that of a single bubble, and increases with the shear rate and the size ratio. The bubble also deforms much more than an isolated one at the same shear rate. Modifying the Chan–Leal solution for the migration of a single submerged bubble or drop, we derive a formula that successfully predicts all the migration trajectories recorded in the experiment. The threshold for migration corresponds to the wall repulsion force overcoming the capillary force in the two-dimensional foam. The size-differentiated bubble migration provides an explanation for previously observed size segregation in sheared three-dimensional polydisperse foams.

We present numerical simulations of a particle trapped at the isotropic-nematic liquid crystal (Iso-N) interface. We use our recent model, based on a phase-field approach [see Qiu et al., Phys. Rev. E 103, 022706 (2021)], to couple the capillary forces acting on the interface with the elastic stresses in the nematic phase along with topological defects. A range of floating configurations are first investigated as a function of the contact angle and various anchoring conditions at the fluid interface. The results show that the response of the system is driven by the existence of an anchoring conflict at the contact line. Substantial particle displacements and/or interfacial deformations may occur in this case even for moderate anchoring strengths. These findings highlight the coupling between elastic and capillary forces. In a second part, we compute drag forces exerted on a particle that moves along the Iso-N interface for several contact angles and a moderate Ericksen number. Because of the coupling between the velocity and order parameter fields, topological defects are swept downstream of the particle by the flow and sometimes escape from the particle or merge with the interface. We also find linear force-velocity laws, with drag forces at the Iso-N interface being slightly greater than their isotropic counterparts due to director distortions. We discuss these results in light of past studies on the behavior of particles being dragged in the bulk of a liquid crystal matrix.

We report an experimental study on shearing a monolayer of monodisperse bubbles floating on liquid in a narrow-gap Couette device. The bubbles in such a “bubble raft” coalesce only if the shear rate exceeds a threshold value. This is in contrast to the conventional wisdom that bubbles and drops coalesce for gentler collisions, at shear rates below a critical value. Furthermore, the threshold shear rate increases with the bubble size and the viscosity of the suspending liquid, contravening reasoning based on capillary number. Through visualization and scaling arguments, we investigate several plausible mechanisms for the anomalous coalescence. None explains all aspects of the observations. The most promising model is one based on inertial forces that compress the bubbles radially inward and accelerate film drainage.

In adopting a poroelastic model for a hydrogel, one views its constituent fluid and solid phases as interpenetrating continua, thereby erasing the pore-scale geometry. This gives rise to the need for additional boundary conditions (BCs) at the interface between a hydrogel and a clear fluid to supplement the momentum equations for the fluid and solid phases in the hydrogel. Using a thermodynamic argument on energy dissipation, we propose three sets of BCs for the gel-fluid interface that link the normal and tangential velocity jumps across the interface to the normal and tangential stresses on either side of the interface. Using several flow problems—one-dimensional compression, two-layer Couette and Poiseuille shear flows, and deformation of a gel particle by a planar extension flow—as tests, we compare the predictions of these three BCs with that of a previously proposed BC. Some differences are stark and reveal flaws in certain BCs. Others are subtler and will require quantitative experimental data for validation. Based on these results, we recommend one set of BCs over the other three for computing the flow and deformation of hydrogels in contact with a clear fluid. In addition, we suggest benchmark experiments to validate the BCs and our recommendation.

Tuning superconductivity in topological materials has been identified as a promising pathway for achieving topological superconductivity, which could accelerate advancements in topological quantum computation. Ternary transition-metal chalcogenides ${ABX}_4$ (where $A/B$ = Zr, Hf, or Ti; $X$ = Te) have been theoretically predicted to be candidates for two-dimensional (2D) topological insulators. In this study we systematically investigate the crystal structure, electronic band properties, and pressure effects of one member of this family, namely, $\mathrm{ZrTiT}{\mathrm{e}}_4$. Our single-crystal x-ray diffraction studies reveal that $\mathrm{ZrTiT}{\mathrm{e}}_4$ adopts a trigonal structure with space group $P\overline{3}m1$ (no. 164), rather than the previously reported monoclinic ($P2/m$) structure. Band structure calculations, complemented by angle-resolved photoemission spectroscopy measurements, indicate that $\mathrm{ZrTiT}{\mathrm{e}}_4$ exhibits a topological semimetal nature. Notably, the application of high pressure induces superconductivity, with a maximum transition temperature ($T_{\mathrm{c}}$) of approximately 5.6 K at pressures around 11–14 GPa. Intriguingly, in the optimal pressure region of the dome-shaped phase diagram, the normal-state resistance shows the highest values, and a nearly linear temperature dependence of resistance is observed above $T_{\mathrm{c}}$. Overall, our results establish the ${ABX}_4$ family as a promising candidate for exploring the interaction between the nontrivial band topology and superconductivity.

Traumatic brain injury (TBI) is a serious health issue. Studies have highlighted the severity of rotation-induced TBI. However, the role of cerebrospinal fluid (CSF) in transmitting the impact from the skull to the soft brain matter remains unclear. Herein, we use experiments and computations to define and probe this role in a simplified setup. A spherical hydrogel ball, serving as a soft brain model, was subjected to controlled rotation within a water bath, emulating the CSF, and filling a transparent cylinder. The cylinder and ball velocities, as well as the ball's deformation over time, were measured. We found that the soft hydrogel ball is very sensitive to decelerating rotational impacts, experiencing significant deformation during the process. A finite-element code is written to simulate the process. The hydrogel ball is modeled as a poroelastic material infused with fluid and its coupling with the suspending fluid is handled by an arbitrary Lagrangian-Eulerian method. The results indicate that the density contrast, as well as the rotational velocity difference, between the hydrogel ball and the suspending fluid, play a central role in the ball's deformation due to centrifugal forces. This approach contributes to a deeper understanding of brain injuries and may portend the development of preventive measures and improved treatment strategies.

The quasi-one-dimensional topological insulator ${\mathrm{TaSe}}_3$ offers an intriguing platform to study the intertwined exotic quantum order, such as superconductivity and charge-density-wave (CDW) order, and nontrivial topological states. Nevertheless, it remains unclear how robust the superconductivity in this quasi-one-dimensional material is and how close the (hidden) CDW exists in its phase diagram. In this work, we unveil a well-defined CDW transition in ${\mathrm{TaSe}}_3$ crystals by leveraging a hydrostatic pressure. Remarkably, it is found that the CDW transition temperature gradually strengthens as the pressure increases, and crosses the room temperature above $\sim 2\mathrm{GPa}$, which makes it promising in future applications in electronic devices. Furthermore, based on the pressure-dependent resistivity and Raman spectroscopy measurements by using a diamond anvil cell, we have extended the high-pressure study up to 50 GPa and verified a proposed phase transition around 6–10 GPa. However, no superconductivity is observed down to 2 K in the whole pressure range studied, suggesting the superconductivity is fragile in this quasi-one-dimensional transition-metal trichalcogenide.

Fiber-based coalescers are widely used to accumulate droplets from aerosols and emulsions, where the accumulated droplets are typically removed by gravity or shear. This Letter reports self-propelled removal of drops from a hydrophobic fiber, where the surface energy released upon drop coalescence overcomes the drop-fiber adhesion, producing spontaneous departure that would not occur on a flat substrate of the same contact angle. The self-removal takes place above a threshold drop-to-fiber radius ratio, and the departure speed is close to the capillary-inertial velocity at large radius ratios.

When exposed to high magnetic fields, certain materials manifest an exotic superconducting (SC) phase that has attracted considerable attention. A proposed explanation for the origin of the high-field SC phase is the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. This state is characterized by inhomogeneous superconductivity, where the Cooper pairs have finite center-of-mass momenta. Recently, the high-field SC phase was observed in FeSe, and it was deemed to originate from the FFLO state. Here, we synthesize FeSe single crystals with different levels of disorder. The level of disorder is expressed by the ratio of the mean free path to the coherence length and ranges between 35 and 1.2. The upper critical field B${}_{\mathrm{c}2}$ was obtained by both resistivity and magnetic torque measurements over a wide range of temperatures, which went as low as $\sim 0.5$ K, and magnetic fields, which went up to $\sim 38$ T along the $c$ axis and in the ab plane. In the high-field region parallel to the ab plane, an unusual SC phase was confirmed in all the crystals, and the phase was found to be robust against disorder. This result suggests that the high-field SC phase in FeSe is not a conventional FFLO state.

Hydrogels consist of a polymer skeleton hydrated by an aqueous solvent, and their hydrodynamics is often described by a coarse-grained poroelasticity model where the boundary conditions between the hydrogel and a surrounding solvent require careful consideration. Young et al. [Phys. Rev. Fluids 4, 063601 (2019)] used the energy dissipation principle to derive a set of boundary conditions regarding the velocity jumps at the interface. However, when applied to an external shear flow over a gel layer, these conditions predict no entrained flow inside the gel, in contrast to the prediction of a previous model by Minale [Phys. Fluids 26, 123102 (2014)]. We adapt the procedure of Young et al. to derive an alternative set of boundary conditions that does allow an external shear flow to induce shear inside the gel and compare the velocity profile to that of Minale. We also derive the limiting form of the boundary conditions in a Darcy medium.

We report observations of topological defects around drops and bubbles that rise through a vertically aligned nematic liquid crystal. We provide direct evidence for downstream convection of the Saturn-ring defect and its transformation to a hyperbolic point defect. The point defect is convected further in the wake of the drop or bubble as the rising velocity increases. In equilibrium, both defect configurations may persist for long times in the narrow cell. But the point defect sometimes spontaneously opens into a Saturn ring, indicating the latter as the globally stable configuration in the presence of tight wall confinement.

The isotropic distributions of projectile fragmentation have been measured for the reactions of 44 MeV/nucleon ${}_{}{}^{129}{}_{}{}^{}\mathrm{Xe}$ on ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$ and ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ targets. The systematics of the production cross sections are discussed via some macroscopic models.

Precision measurements of cosmic ray positrons are presented up to 1 TeV based on 1.9 million positrons collected by the Alpha Magnetic Spectrometer on the International Space Station. The positron flux exhibits complex energy dependence. Its distinctive properties are (a) a significant excess starting from $25.2\pm 1.8\mathrm{GeV}$ compared to the lower-energy, power-law trend, (b) a sharp dropoff above $284_{-64}^{+91}\mathrm{GeV}$, (c) in the entire energy range the positron flux is well described by the sum of a term associated with the positrons produced in the collision of cosmic rays, which dominates at low energies, and a new source term of positrons, which dominates at high energies, and (d) a finite energy cutoff of the source term of $E_s=810_{-180}^{+310}\mathrm{GeV}$ is established with a significance of more than $4\sigma$. These experimental data on cosmic ray positrons show that, at high energies, they predominantly originate either from dark matter annihilation or from other astrophysical sources.

We report the observation of new properties of primary cosmic rays He, C, and O measured in the rigidity (momentum/charge) range 2 GV to 3 TV with $90\times {10}^6$ helium, $8.4\times {10}^6$ carbon, and $7.0\times {10}^6$ oxygen nuclei collected by the Alpha Magnetic Spectrometer (AMS) during the first five years of operation. Above 60 GV, these three spectra have identical rigidity dependence. They all deviate from a single power law above 200 GV and harden in an identical way.

A precision measurement by AMS of the positron fraction in primary cosmic rays in the energy range from 0.5 to 500 GeV based on 10.9 million positron and electron events is presented. This measurement extends the energy range of our previous observation and increases its precision. The new results show, for the first time, that above $\sim 200\mathrm{GeV}$ the positron fraction no longer exhibits an increase with energy.

We searched for proton decay via $p\to {\mu }^{+}{K}^0$ in $0.37\mathrm{Mton}·\mathrm{years}$ of data collected between 1996 and 2018 from the Super-Kamiokande water Cherenkov experiment. The selection criteria were defined separately for $K_S^0$ and $K_L^0$ channels. No significant event excess has been observed. As a result of this analysis, which extends the previous search by an additional $0.2\mathrm{Mton}·\mathrm{years}$ of exposure and uses an improved event reconstruction, we set a lower limit of $3.6\times {10}^{33}\mathrm{years}$ on the proton lifetime.

We report the properties of primary cosmic-ray sulfur (S) in the rigidity range 2.15 GV to 3.0 TV based on $0.38\times {10}^6$ sulfur nuclei collected by the Alpha Magnetic Spectrometer experiment (AMS). We observed that above 90 GV the rigidity dependence of the S flux is identical to the rigidity dependence of Ne-Mg-Si fluxes, which is different from the rigidity dependence of the He-C-O-Fe fluxes. We found that, similar to N, Na, and Al cosmic rays, over the entire rigidity range, the traditional primary cosmic rays S, Ne, Mg, and C all have sizeable secondary components, and the S, Ne, and Mg fluxes are well described by the weighted sum of the primary silicon flux and the secondary fluorine flux, and the C flux is well described by the weighted sum of the primary oxygen flux and the secondary boron flux. The primary and secondary contributions of the traditional primary cosmic-ray fluxes of C, Ne, Mg, and S (even $Z$ elements) are distinctly different from the primary and secondary contributions of the N, Na, and Al (odd $Z$ elements) fluxes. The abundance ratio at the source for $\mathrm{S}/\mathrm{Si}$ is $0.167\pm 0.006$, for $\mathrm{Ne}/\mathrm{Si}$ is $0.833\pm 0.025$, for $\mathrm{Mg}/\mathrm{Si}$ is $0.994\pm 0.029$, and for $\mathrm{C}/\mathrm{O}$ is $0.836\pm 0.025$. These values are determined independent of cosmic-ray propagation.