Fluid-like State Research Articles

Lateral Spreading in Christchurch, New Zealand: An Empirical Approach

Lateral spreading, a complex phenomenon resulting from liquefaction, manifests when saturated, cohesionless soils lose their strength during seismic events, causing them to deform and flow horizontally. This process poses a substantial risk to buildings and infrastructure, often resulting in extensive damage, significant financial burdens, and, tragically, loss of life. In Indonesian, liquefaction is recognized for its ability to transform solid ground into a fluid-like state, amplifying its danger in regions prone to earthquakes. This study aims to analyze lateral spreading through empirical methods, specifically employing the Bartlett & Youd Method (2002) and the Byrne Method (1990). The analysis focuses on sites previously affected by lateral spreading, notably those impacted by the 22 February 2011 Christchurch earthquake, which registered a magnitude of 6.2 and a peak ground acceleration of 0.52. The selected locations include the South Brighton Bridge, Anzac Bridge, and Fitzgerald Bridge in Christchurch, New Zealand. The findings demonstrate that both the Bartlett & Youd Method (2002) and the Byrne Method (1990) yield results that closely approximate the actual conditions at site.

Dynamic regulation of tissue fluidity controls skin repair during wound healing

During wound healing, different pools of stem cells (SCs) contribute to skin repair. However, how SCs become activated and drive the tissue remodeling essential for skin repair is still poorly understood. Here, by developing a mouse model allowing lineage tracing and basal cell lineage ablation, we monitor SC fate and tissue dynamics during regeneration using confocal and intravital imaging. Analysis of basal cell rearrangements shows dynamic transitions from a solid-like homeostatic state to a fluid-like state allowing tissue remodeling during repair, as predicted by a minimal mathematical modeling of the spatiotemporal dynamics and fate behavior of basal cells. The basal cell layer progressively returns to a solid-like state with re-epithelialization. Bulk, single-cell RNA, and epigenetic profiling of SCs, together with functional experiments, uncover a common regenerative state regulated by the EGFR/AP1 axis activated during tissue fluidization that is essential for skin SC activation and tissue repair.

A shape-driven reentrant jamming transition in confluent monolayers of synthetic cell-mimics

Many critical biological processes, like wound healing, require densely packed cell monolayers/tissues to transition from a jammed solid-like to a fluid-like state. Although numerical studies anticipate changes in the cell shape alone can lead to unjamming, experimental support for this prediction is not definitive because, in living systems, fluidization due to density changes cannot be ruled out. Additionally, a cell’s ability to modulate its motility only compounds difficulties since even in assemblies of rigid active particles, changing the nature of self-propulsion has non-trivial effects on the dynamics. Here, we design and assemble a monolayer of synthetic cell-mimics and examine their collective behaviour. By systematically increasing the persistence time of self-propulsion, we discovered a cell shape-driven, density-independent, re-entrant jamming transition. Notably, we observed cell shape and shape variability were mutually constrained in the confluent limit and followed the same universal scaling as that observed in confluent epithelia. Dynamical heterogeneities, however, did not conform to this scaling, with the fast cells showing suppressed shape variability, which our simulations revealed is due to a transient confinement effect of these cells by their slower neighbors. Our experiments unequivocally establish a morphodynamic link, demonstrating that geometric constraints alone can dictate epithelial jamming/unjamming.

Perspectives in collective cell migration - moving forward.

Collective cell migration, where cells move as a cohesive unit, is a vital process underlying morphogenesis and cancer metastasis. Thanks to recent advances in imaging and modelling, we are beginning to understand the intricate relationship between a cell and its microenvironment and how this shapes cell polarity, metabolism and modes of migration. The use of biophysical and mathematical models offers a fresh perspective on how cells migrate collectively, either flowing in a fluid-like state or transitioning to more static states. Continuing to unite researchers in biology, physics and mathematics will enable us to decode more complex biological behaviours that underly collective cell migration; only then can we understand how this coordinated movement of cells influences the formation and organisation of tissues and directs the spread of metastatic cancer. In this Perspective, we highlight exciting discoveries, emerging themes and common challenges that have arisen in recent years, and possible ways forward to bridge the gaps in our current understanding of collective cell migration.

A perspective on active glassy dynamics in biological systems

AbstractDynamics is central to living systems. Many experiments in the last two decades have revealed glassy dynamics in diverse biological systems, showing a transition between a solid-like and a fluid-like state. The biological systems have nontrivial characteristics: they are active with novel control parameters and immense complexity. Moreover, glassiness in these systems has many nontrivial features, such as the behavior of dynamical heterogeneity and readily found sub-Arrhenius relaxation dynamics. Theoretical treatments of these systems are generally challenging due to their nonequilibrium nature and large number of control parameters. We first discuss the primary characteristics of a glassy system and then review the experiments that started this field and simulations that have led to a deeper understanding. We also show that despite many challenges in these systems, it has been possible to develop theories that have played a significant role in unifying diverse phenomena and bringing insights. The field is at the interface of physics and biology, freely borrowing tools from both disciplines. We first discuss the known equilibrium scenario and then present the primary changes under activity.

Subgrade soil response to rail loading: Instability mechanisms, causative factors, and preventive measures

The demand for more efficient heavy-haul rail networks over soft subgrades poses significant geotechnical challenges and requires a comprehensive understanding of stress conditions as well as the failure potential of subgrade soil under moving wheel loads and increasing rail speeds. Unfavourable stress conditions in the subgrade can result in various types of failures, three of which are identified in this article: (i) excessive plastic settlement, (ii) progressive shear failure, and (iii) subgrade fluidisation (mud pumping). Through a series of advanced testing schemes using cyclic triaxial, hollow cylinder, and an in-house dynamic filtration apparatus, critical stress conditions and soil characteristics prone to subgrade instability are discussed. The results demonstrate that under adverse combinations of loading frequency (f) and cyclic stress ratio (CSR) the continuous application of cyclic loads can lead to an unstable state of soil where excess pore pressure and axial strain increase rapidly. This study also reveals that low to medium plasticity soils (PI < 22) are more vulnerable to subgrade fluidisation, where the rapid internal migration of pore water transforms the upper soil to a fluid-like state with substantial loss in soil stiffness. The layered response of soil through dynamic filtration tests showed larger hydrodynamic forces induced by differential hydraulic gradients in the top layer during cyclic loading causes moisture to move upwards. Various factors that can influence soil instability such as the degree of compaction degree, clay content, soil fabric and stress rotation are also addressed in this paper. Finally, novel solutions for stabilising subgrade such as a vertical drain-composite system and the use of eco-friendly biopolymers are presented.

Effects of sugar molecules on the rheological and tribological properties and on the microstructure of agarose-based fluid gels

Introduction: Fluid gels exhibit a unique flow behaviour. Their pronounced viscoelastic behaviour arises from irregular microgel particles, leading to plasticity and yielding, as well as a characteristic transition from a solid to a fluid-like state. This is defined by both the core and the surface structures of the particles. Adding co-solutes such as sucrose alters the network properties at the molecular scale, affecting texture and lubrication. This study investigates how the microscopic changes due to sucrose addition correlate with macroscopic changes.Material and methods: Agarose fluid gels with varying sucrose concentrations were prepared and studied using the rheometer. Temperature dependent viscosity behaviour during gelation under shear was investigated in situ. Light microscopy and particle size measurements were examined, and complemented by amplitude, frequency and flow sweeps as well as tribological studies. These tests allow us to understand the influence of sucrose on the particle network formation.Results and discussion: Sucrose concentration influenced the sizes, shapes, and interconnected network structure of the microgel particles. These microstructural changes are closely related to the dynamic competition between gelation and disruption of the agarose chains during shear, which directly influences the rheological and tribological properties of the resulting fluid gels. Additionally, the association of the agarose chains and their gelation process is also influenced depending on whether the sucrose or agarose was first dissolved in water. The experimental observations suggest specific molecular mechanisms, explaining the role of sucrose in structure formation of agarose-based fluid gels. These results have the potential to expand the applications of fluid gels, which play a crucial role in modifying the texture and flow behaviour of foods and beverages, particularly in addressing challenges such as dysphagia.

Evaluation on pH-dependent thermal gelation performance of chickpea, pea protein, and casein micelles

Concerns about health, the environment, and sustainable food supply have inspired researchers into searching new alternative plant proteins to dairy proteins. This study investigated thermal treatment coupled with pH manipulation (3.0, 5.0, 7.0, and 9.0) on the structural and gelation performance of pea, chickpea protein, and casein. Gel electrophoresis suggested that only specific fractions contributed to disulfide bond-mediated aggregates formation: bovine κ-casein and αs2-casein in casein micelles, acidic and basic legumin subunits in pea and chickpea protein. No protein formed self-standing gels but precipitated in macroscopic flocs at pH 5.0. Casein displayed optimum gel performance at pH 3.0 (storage modulus (G′) of 2596.00 Pa, hardness of 603.47 g, water holding capacity of 94.92%), and transitioned into fluid-like viscous state (G′ < 1, loss tangent >1) under neutral or higher pH conditions. Pea protein did not form self-holding gels but aggregates with globular particles over all test pH. Nevertheless, chickpea protein formed self-standing gels regardless of pH (except 5.0) and peaked gel performance at pH 9.0. Hence, this study indicated considerable potential of casein in acidic thermal gel-based food products and confirmed the promising applications of chickpea protein as an alternative to pea protein across diverse pH conditions, and to casein in neutral and alkaline food gel formulations. In addition to providing scientific insight understanding the effects of thermal treatment coupled with pH manipulation on the performance of plant-based protein and dairy protein, this research sheds light on the promising prospects of incorporating chickpea protein into gel-based food formulations.

Investigation on tectonically deformed coal particle crushing by DEM under triaxial loading: Implication for coal and gas outburst

Tectonically deformed coal is composed of loosely bonded and brittle particles. The effect of particle crushing on tectonically deformed coal was studied through triaxial loading tests using the discrete element method. Coal particles are highly sensitive to crushing under confining pressure. When confined pressure at 2.5 MPa, the sample behaves similarly to an uncrushed sample. The majority of boundary work is consumed by particle friction and crushing during loading. The porosity increases obviously, the granular system is transferred into fluid-like state. With an increase in confining pressure, particle crushing intensifies, resulting in a growth in weak force chains and crushing energy, and a decrease in porosity. The pattern of particle crushing is closely linked to specific force chains. Particle rotation is a significant factor affecting particle crushing, and it hinders the connection of shear zones. Crushed particles decrease porosity by filling the gaps between larger particles. After compaction, crushed particles form a crushing and compaction belt, which creates local gas sealing conditions and makes gas extraction difficult in tectonically deformed coal areas. These characteristics play a crucial role in understanding how in-situ stress impacts gas outbursts and the difficulty of gas extraction.

Peridynamic mesh-free simulation of glass and metal beads column collapses

The density of the metal beads is almost 3 times the density of glass beads, and the rheological response is consequently different from glass beads by using the μ(I) rheology. In this study, the μ(I) rheology is first applied to simulate metal beads column collapse, which is implemented in Peridynamics to capture the transition from the fluid-like state to the solid-like state in the granular flow. To achieve this, flow response by the rheological parameters in the μ(I) rheology is extensively investigated by simulating glass beads column collapse. The numerical results are compared to the experimental measurements in terms of the free surface, velocity distribution, and the interface in the collision of two adjacent granular columns. It is found that the mobility of the granular material is significantly affected by static friction μs. The friction μ2 with high inertia in the rheology can affect the movements of free surface points in the simulations. Another parameter of a constant I0 in the rheology can reduce the material's mobility if its value is very small and large values show insignificant effects on the flow characteristics. The parameters of (μs, μ2, I0) = (0.36, 0.42, 0.01) in the rheology model can capture the free surface profiles in good agreement with the experimental measurements for the metal beads column collapse with different aspect ratios by using the numerical method.

Cell jamming regulates epithelial chiral morphogenesis

Internal organs such as the heart demonstrate apparent left–right (LR) asymmetric morphology and positioning. Cellular chirality and associated LR biased mechanical behavior such as cell migration have been attributed to LR symmetry breaking during embryonic development. Mathematical models have shown that chiral directional migration can be driven by cellular intrinsic torque. Tissue jamming state (i.e., solid-like vs fluid-like state) strongly regulates collective migratory behavior, but how it might affect chiral morphogenesis is still unknown. Here, we develop a cell vertex model to study the role of tissue rigidity or jamming state on chiral morphogenesis of the cells on a patterned ring-shaped tissue, simulating a previously reported experimental setup for measuring cell chirality. We simulate chirality as torsional forces acting on cell vertices. As expected, the cells undergo bidirectional migration at the opposing (inner and outer) boundaries of the ring-shaped tissue. We discover that more fluid-like tissues (unjammed) demonstrate a stronger chiral cell alignment and elongation than more solid-like (jammed) tissues and maintain a bigger difference in migration velocity between opposing tissue boundaries. Finally, we find that fluid-like tissues undergo more cell-neighbor exchange events. This study reveals that chiral torque is sufficient to achieve a biased cellular alignment as seen in vitro. It further sheds light on the mechanical regulation of chiral morphogenesis of tissues and reveals a role of cell density-independent tissue rigidity in this process.

Jamming Transitions in Astrocytes and Glioblastoma Are Induced by Cell Density and Tension

Collective behavior of cells emerges from coordination of cell-cell-interactions and is important to wound healing, embryonic and tumor development. Depending on cell density and cell-cell interactions, a transition from a migratory, fluid-like unjammed state to a more static and solid-like jammed state or vice versa can occur. Here, we analyze collective migration dynamics of astrocytes and glioblastoma cells using live cell imaging. Furthermore, atomic force microscopy, traction force microscopy and spheroid generation assays were used to study cell adhesion, traction and mechanics. Perturbations of traction and adhesion were induced via ROCK or myosin II inhibition. Whereas astrocytes resided within a non-migratory, jammed state, glioblastoma were migratory and unjammed. Furthermore, we demonstrated that a switch from an unjammed to a jammed state was induced upon alteration of the equilibrium between cell-cell-adhesion and tension from adhesion to tension dominated, via inhibition of ROCK or myosin II. Such behavior has implications for understanding the infiltration of the brain by glioblastoma cells and may help to identify new strategies to develop anti-migratory drugs and strategies for glioblastoma-treatment.

Three-dimensional printing of soft hydrogel electronics

Electronics based on hydrogels can have inherent similarities to biological tissue and are of potential use in biomedical applications. Ideally, such hydrogel electronics should offer customizable three-dimensional circuits, but making complex three-dimensional circuits encapsulated within a hydrogel matrix is challenging with existing materials and manufacturing methods. Here we report the three-dimensional printing of hydrogel electronics using a curable hydrogel-based supporting matrix and a stretchable silver–hydrogel ink. The supporting matrix has a yield stress fluid behaviour, so the shear force generated by a moving printer nozzle creates a temporary fluid-like state, allowing the accurate placement in the matrix of silver–hydrogel ink circuits and electronic components. After printing, the entire matrix and embedded circuitry can be cured at 60 °C to form soft (Young’s modulus of less than 5 kPa) and stretchable (elongation of around 18) monolithic hydrogel electronics, whereas the conductive ink exhibits a high conductivity of around 1.4 × 103 S cm−1. We use our three-dimensional printing approach to create strain sensors, inductors and biological electrodes.

Local Yield and Compliance in Active Cell Monolayers.

The rheology of biological tissue plays an important role in many processes, from organ formation to cancer invasion. Here, we use a multiphase field model of motile cells to simulate active microrheology within a tissue monolayer. When unperturbed, the tissue exhibits a transition between a solidlike state and a fluidlike state tuned by cell motility and deformability-the ratio of the energetic costs of steric cell-cell repulsion and cell-edge tension. When perturbed, solid tissues exhibit local yield-stress behavior, with a threshold force for the onset of motion of a probe particle that vanishes upon approaching the solid-to-liquid transition. This onset of motion is qualitatively different in the low and high deformability regimes. At high deformability, the tissue is amorphous when solid, it responds compliantly to deformations, and the probe transition to motion is smooth. At low deformability, the monolayer is more ordered translationally and stiffer, and the onset of motion appears discontinuous. Our results suggest that cellular or nanoparticle transport in different types of tissues can be fundamentally different and point to ways in which it can be controlled.

Liquefaction of granular materials in constant-volume cyclic shearing: Transition between solid-like and fluid-like states

By means of 3D-DEM cyclic simple shear simulations at constant volume, this paper analyzes the evolution of the internal structure during cyclic liquefaction of dense granular media. Upon liquefaction, the granular system undergoes a fast transition to a regime where the fluid-like shear strain develops. This regime is characterized by low shear modulus, decreasing dilatancy, and reduced shear viscosity. We analyze the internal structure by means of several micro- and meso-scale descriptors, including coordination number, fabric anisotropy, number of clusters, a fraction of maximum cluster size, and a percolation index. We select a typical post-liquefaction cycle in which we highlight several particular states that naturally divide the cycle into different periods. During liquefaction, the system deforms significantly with an enhanced number of binary collisions initially and particle clusters subsequently. Both the coordination number and percolation index increase while fabric anisotropy oscillates. The fluid-like to solid-like transition is characterized by a well-defined value of the coordination number (3.6) when the particles percolate across the system. From the analysis of the micro- and meso-scale behaviors, we further discuss two possible criteria for exiting the fluid-like state based on the excess pore pressure.

Localized behaviour of fluidized subgrade soil subjected to cyclic loading

Recent investigations have shown that under adverse cyclic triaxial loading, the upper part of soil specimens can turn into a fluid-like state with increased water content (i.e., fluidization), whereas the lower layers can maintain a relatively high stiffness. This paper aims to gain further insight into this behaviour by monitoring the development in excess pore water pressure (EPWP) at the top and bottom of the test specimens, followed by post-analysis of water content distribution along the specimen. The results show that the EPWP at the uppermost part of the specimen develops rapidly and approaches the zero-effective stress level, whereas the EPWP at the bottom part of the specimen tends to stabilize while undergoing densification. Accompanied with this process is a redistribution of the water content along the specimen height where the water content at the upper soil layer increases to approach the liquid limit while increasing the void ratio.

Quantum phases and spin liquid properties of 1T-TaS2

Quantum materials exhibiting magnetic frustration are connected to diverse phenomena, including high Tc superconductivity, topological order, and quantum spin liquids (QSLs). A QSL is a quantum phase (QP) related to a quantum-entangled fluid-like state of matter. Previous experiments on QSL candidate materials are usually interpreted in terms of a single QP, although theories indicate that many distinct QPs are closely competing in typical frustrated spin models. Here we report on combined temperature-dependent muon spin relaxation and specific heat measurements for the triangular-lattice QSL candidate material 1T-TaS2 that provide evidence for competing QPs. The measured properties are assigned to arrays of individual QSL layers within the layered charge density wave structure of 1T-TaS2 and their characteristic parameters can be interpreted as those of distinct Z2 QSL phases. The present results reveal that a QSL description can extend beyond the lowest temperatures, offering an additional perspective in the search for such materials.

IR detection of the methane halides fluid-like state at ambient conditions

The paper presents the IR-study of the fluid-like state, generated at ambient conditions, for methane halides (iodomethane, tetrachloromethane, trichloromethane and dichloromethane). It was shown that at vapor compressionextension procedure realized in variable thickness spectral cell (VTOC), the dual phase state exhibiting both gas and liquid properties arises. A reversible transition from a gas-like to a liquid-like shape, independent on the thermodynamic characteristics of the studied methane halides, was revealed.

Preliminary Study on Fluidized Bed Chemical Mechanical Polishing (FB-CMP) Process for Stainless Steel 304 (SS304).

Fluidized bed machining (FBM) is used for the surface finishing or cleaning of complex 3D machine parts. FBM is a process of injecting air into a chamber to encourage particles into a fluid-like state. Subsequently, FBM involves rotating the specimen at high speed to process the surface of the material. However, owing to the long processing time involved in FBM, there is a limit to its application in various industries. In this paper, we propose a fluidized bed chemical mechanical polishing (FB-CMP) process, wherein the material removal mechanism of chemical mechanical polishing (CMP) is applied to FBM to improve the processing efficiency of FBM. An FB-CMP system was prepared, and preliminary experiments on the chemical solution were conducted using stainless steel 304 (SS304) plates. In the experiment, hydrogen peroxide (H2O2) was used as the oxidant, oxalic acid (C2H2O4) was used as the complexing agent and alumina (Al2O3) was used as the abrasive particle. The material removal rate (MRR) and roughness reduction rate during the FB-CMP of SS304 were dependent on the composition of the chemical solution. The experimental results revealed the highest MRR and roughness reduction rate at 0.33 wt % H2O2 and 0.2 wt % oxalic acid. To stabilize the proposed FB-CMP process, it is necessary to examine the chemical solutions of various materials.

A curing model for the numerical simulation within additive manufacturing of soft polymers using peridynamics

Within this paper, the modelling and simulation of extrusion-based Additive Manufacturing (AM) processes of curing polymers is presented. The challenge of the AM is the adjustment of processing parameters. This includes the application of laser radiation to locally accelerate the curing in order to control the final geometry of the implant. Since complex multi-physical coupling effects are hardly predictable by operator experience, numerical simulations are beneficial. When the underlying physical effects of the AM processes are captured correctly within the simulations, a realistic representation of the process is possible. To model the material behaviour during the process, a process-dependent large strain curing model is formulated, considering the stress free curing behaviour of the material. State-of-the-art models are not able to model the fluid-like behaviour of low cured polymers. This needs a formulation that takes into account finite deformations. Hence, the current model is extended to finite plasticity using a process-dependent yield function. This allows the modelling of material spreading in the fluid-like state by simultaneously reducing the accumulation of elastic stored energy, which would lead to an unintentional and non-physical bounce-off behaviour otherwise. For the numerical simulations, an enhanced version of the peridynamic correspondence formulation using fractional subfamilies with associated volume weighting factors is introduced and implemented. Besides the specific laser modelling as a volumetric heat source, a local–non-local coupling of the arising thermo-chemo-mechanical coupled equations is introduced within the peridynamic framework. Within the simulations, the applicability of the plasticity-based approach to model material spreading in the fluid-like state is presented. Finally, the software for extrusion-based printing processes is developed and the complete thermo-chemo-mechanical coupled AM process is simulated. It is shown that higher geometrical precision is obtainable in terms of a reduced material spreading by the application of a laser radiation. The model constitutes the first step of the virtual implant development regarding the optimisation possibilities during the AM process.