Soft Glassy Materials Research Articles

Understanding the effect of physical aging on slip dynamics in soft-glassy materials using pure elongation flow

Slip over a solid surface is a very common occurrence in industrial scale transport and the processing of complex fluids. The knowledge of slip also plays a huge role in the correct estimation of rheological properties. In this work, we have studied the slip dynamics in a model soft-glassy material that exhibits physical aging, wherein the structure evolves gradually toward a more solid-like character via rearrangement of constituents. More precisely, we have investigated the impact of physical aging on slip associated with pure elongation flow of the material. We have allowed the sample to age over different waiting times, followed by the sample being deformed slowly in elongation mode by pulling the top plate to achieve a pure elongation flow. Normal force as a function of gap has been recorded during such pure elongation. These normal force–gap curves demonstrated a remarkable gap-waiting time superposition, manifesting the strong signature of self-similarity in the pure elongation flow of the soft-glassy system. We have adopted a slip layer model, which predicted these normal force–gap flow curves remarkably well. Such prediction also rendered slip layer thickness as a function of waiting time, using which we have explained the intriguing self-similar nature of normal force–gap dependence. Finally, we have established a relationship between the slip layer thickness and the age-dependent bulk rheological properties. We have provided a possible physical reasoning to explain this link between the physical aging-driven state of material and the slip dynamics.

Universal mechanism of shear thinning in supercooled liquids

Soft glassy materials experience a significant reduction in viscosity η when subjected to shear flow, known as shear thinning. This phenomenon is characterized by a power-law scaling of η with the shear rate γ°\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\dot{\\gamma }$$\\end{document}, η∝γ°−ν\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\eta \\propto {\\dot{\\gamma }}^{-\ u }$$\\end{document}, where the exponent ν is typically around 0.7 to 0.8 across different materials. Two decades ago, the mode-coupling theory (MCT) suggested that shear thinning occurs due to the advection. However, it predicts too large ν = 1 ( > 0.7 to 0.8) and overestimates the onset shear rate by orders of magnitude. Recently, it was claimed that a minute distortion of the particle configuration is responsible for shear thinning. Here we extend the MCT to include the distortion, and find that both advection and distortion contribute to shear thinning, but the latter is dominant. Our formulation works quantitatively for several different glass formers. We explain why shear thinning is universal for many glassy materials.

Soft glassy materials with tunable extensibility.

Extensibility is beyond the paradigm of classical soft glassy materials, and more broadly, yield-stress fluids. Recently, model yield-stress fluids with significant extensibility have been designed by adding polymeric phases to classically viscoplastic dispersions [Nelson et al., J. Rheol., 2018, 62, 357; Nelson et al., Curr. Opin. Solid State Mater. Sci., 2019, 23, 100758; Dekker et al., J. Non-Newtonian Fluid Mech., 2022, 310, 104938]. However, fundamental questions remain about the design of and coupling between the shear and extensional rheology of such systems. In this work, we propose a model material, a mixture of soft glassy microgels and solutions of high molecular weight linear polymers. We establish systematic criteria for the design and thorough rheological characterization of such systems, in both shear and extension. Using our material, we show that it is possible to dramatically change the behavior in extension with minimal change in the shear yield stress and elastic modulus, thus enabling applications that exploit orthogonal modulation of shear and extensional material properties.

Rheology of edible soft glassy materials

In this paper we investigate the rheology of three different classes of edible yield stress fluids: a) cookie doughs, b) vegetable/fruit purees, and c) protein-rich doughs as used in meat analogs and 3D food printing. Strains sweeps of different formulations within one class can be mapped to a (partial) master curve, with at least the same strain softening index, and a similar crossover point. For classes showing only a partial master curve, the formulation differs in the response of the loss modulus in the linear viscoelastic (LVE) regime, and the transition regime between LVE and strain softening regimes, where several formulations also show a weak strain overshoot. Amongst different classes, different behaviour in strain softening has been observed. Meat analogs and fruit purees show a ratio of 2 between strain softening indices of elastic and loss modulus, while for cookie doughs and protein-rich inks, the ratio is near unity. A constitutive model using a configurational tensor with shear rate-dependent viscosity and strain-dependent elastic modulus can reproduce all features observed for the different classes and for the various formulations within a class. We conclude that our constitutive model is a good basis to design edible yield stress fluids for fiber-enrichment, starch and/or sugar replacement.

Anticipating gelation and vitrification with medium amplitude parallel superposition (MAPS) rheology and artificial neural networks

Anticipating qualitative changes in the rheological response of complex fluids (e.g., a gelation or vitrification transition) is an important capability for processing operations that utilize such materials in real-world environments. One class of complex fluids that exhibits distinct rheological states are soft glassy materials such as colloidal gels and clay dispersions, which can be well characterized by the soft glassy rheology (SGR) model. We first solve the model equations for the time-dependent, weakly nonlinear response of the SGR model. With this analytical solution, we show that the weak nonlinearities measured via medium amplitude parallel superposition (MAPS) rheology can be used to anticipate the rheological aging transitions in the linear response of soft glassy materials. This is a rheological version of a technique called structural health monitoring used widely in civil and aerospace engineering. We design and train artificial neural networks (ANNs) that are capable of quickly inferring the parameters of the SGR model from the results of sequential MAPS experiments. The combination of these data-rich experiments and machine learning tools to provide a surrogate for computationally expensive viscoelastic constitutive equations allows for rapid experimental characterization of the rheological state of soft glassy materials. We apply this technique to an aging dispersion of Laponite® clay particles approaching the gel point and demonstrate that a trained ANN can provide real-time detection of transitions in the nonlinear response well in advance of incipient changes in the linear viscoelastic response of the system.

Rheological signatures of a glass-glass transition in an aging colloidal clay

The occurrence of non-equilibrium transitions between arrested states has recently emerged as an intriguing issue in the field of soft glassy materials. The existence of one such transition has been suggested for aging colloidal clays (Laponite® suspensions) at a weight concentration of 3.0%, although further experimental evidences are necessary to validate this scenario. Here, we test the occurrence of this transition for spontaneously aged (non-rejuvenated) samples by exploiting the rheological tools of dynamical mechanical analysis. On imposing consecutive compression cycles to differently aged clay suspensions, we find that quite an abrupt change of rheological parameters occurs for ages around three days. For Young’s and elastic moduli, the change with the waiting time is essentially independent from the deformation rate, whereas other “fluid-like” properties, such as the loss modulus, do clearly display some rate dependence. We also show that the crossover identified by rheology coincides with deviations of the relaxation time (obtained through x-ray photon correlation spectroscopy) from its expected monotonic increase with aging. Thus, our results robustly support the existence of a glass-glass transition in aging colloidal clays, highlighting characteristic features of their viscoelastic behavior.

Evolution of flow reversal and flow heterogeneities in high elasticity wormlike micelles (WLMs) with a yield stress

The formation and evolution of a heterogeneous flow and flow reversal are examined in highly elastic, gel-like wormlike micelles (WLMs) formed from an amphiphilic triblock poloxamer P234 in 2M NaCl. A combination of linear viscoelastic, steady shear, and creep rheology demonstrate that these WLMs have a yield stress and exhibit viscoelastic aging, similar to some soft glassy materials. Nonlinear shear rheology and rheoparticle tracking velocimetry reveal that these poloxamer WLMs undergo a period of strong elastic recoil and flow reversal after the onset of shear startup. As flow reversal subsides, a fluidized high shear rate region and a nearly immobile low shear rate region of fluid form, accompanied by wall slip and elastic instabilities. The features of this flow heterogeneity are reminiscent of those for aging yield stress fluids, where the heterogeneous flow forms during the initial stress overshoot and is sensitive to the inherent stress gradient of the flow geometry. Additionally, macroscopic bands that form transiently above a critical shear rate become “trapped” due to viscoelastic aging in the nearly immobile region. This early onset of the heterogeneous flow during the rapidly decreasing portion of the stress overshoot differs from that typically observed in shear banding WLMs and is proposed to be necessary for observing significant flow reversal. Exploring the early-time, transient behavior of this WLM gel with rheology similar to both WLM solutions and soft glassy materials provides new insights into spatially heterogeneous flows in both of these complex fluids.

Introduction to viscoelasticity and plasticity, and their relation to the underlying microscopic dynamics in soft matter systems

Understanding the rheological behavior of complex fluids, and in particular its relation to the dynamics of the elementary microscopic constituents, is one of the open challenges in soft matter science. In this contribution, we give a brief introduction to linear shear rheology and passive microrheology of soft amorphous materials, and we then focus on the transition to non-linearity during oscillatory rheology tests performed on soft glassy materials. These materials exhibit solid-like properties at rest, but they display characteristics of fluids when a sufficiently large shear is applied. Significant progress in understanding such ‘fluidization’ has been recently achieved by combining rheometry with techniques capable to capture the microscopic dynamics induced by shear, and in particular by oscillatory shear. We here summarize some of these achievements, as an introduction to the field for a non-expert reader.

Continuum modeling of soft glassy materials under shear

Soft Glassy Materials (SGM) consist in dense amorphous assemblies of colloidal particles of multiple shapes, elasticity, and interactions, which confer upon them solid-like properties at rest. They are ubiquitously encountered in modern engineering, including additive manufacturing, semi-solid flow cells, dip coating, adhesive locomotion, where they are subjected to complex mechanical histories. Such processes often include a solid-to-liquid transition induced by large enough shear, which results in complex transient phenomena such as non-monotonic stress responses, i.e., stress overshoot, and spatially heterogeneous flows, e.g., shear banding or brittle failure. In the present article, we propose a pedagogical introduction to a continuum model based on a spatially resolved fluidity approach that we recently introduced to rationalize shear-induced yielding in SGMs. Our model, which relies upon non-local effects, quantitatively captures salient features associated with such complex flows, including the rate dependence of the stress overshoot, as well as transient shear-banded flows together with non-trivial scaling laws for fluidization times. This approach offers a versatile framework to account for subtle effects, such as avalanche-like phenomena, or the impact of boundary conditions, which we illustrate by including in our model the elasto-hydrodynamic slippage of soft particles compressed against solid surfaces.

Experimental observations of fractal landscape dynamics in a dense emulsion.

Many soft and biological materials display so-called 'soft glassy' dynamics; their constituents undergo anomalous random motions and complex cooperative rearrangements. A recent simulation model of one soft glassy material, a coarsening foam, suggested that the random motions of its bubbles are due to the system configuration moving over a fractal energy landscape in high-dimensional space. Here we show that the salient geometrical features of such high-dimensional fractal landscapes can be explored and reliably quantified, using empirical trajectory data from many degrees of freedom, in a model-free manner. For a mayonnaise-like dense emulsion, analysis of the observed trajectories of oil droplets quantitatively reproduces the high-dimensional fractal geometry of the configuration path and its associated local energy minima generated using a computational model. That geometry in turn drives the droplets' complex random motion observed in real space. Our results indicate that experimental studies can elucidate whether the similar dynamics in different soft and biological materials may also be due to fractal landscape dynamics.

Flow of wormlike micelles: From shear banding to elastic turbulence

Shear banding is a ubiquitous phenomenon in complex fluids flows. Most often it corresponds to a flow state in which a fluid splits into layers of differing apparent viscosities supporting different local shear rates. Shear banding has been observed, either transiently or at steady state, in different classes of complex systems including polymeric fluids and soft glassy materials. Its description and understanding is particularly advanced in surfactant wormlike micelles, for which the experimental evidence of its mechanical signature dates back to the 1990s. Over the past 10 years a strong effort was made to combine global rheology with local time-resolved techniques to probe both the velocity field and the structural properties in various macro- and micro-fluidic geometries. Overall, complex flows have been observed to emerge on top of the shear-banding flow and have been connected to the development of viscoelastic instabilities.In this talk I will try to describe the complete picture of the shear-banding transition in wormlike micelles flowing in Taylor-Couette geometry, from the onset of banding to the development of elastic turbulence. The response of shear-banding wormlike to time-dependent flow protocols such as step stress and shear startup will be examined. The mechanical signatures of the onset of banding will be discussed and compared with general criteria derived by Moorcroft and Fielding. Then I will describe the spatio-temporal dynamics over longer time scales showing the development of secondary flows on top of shear banding. I will also address the transition towards a disordered flow state reminiscent of elastic turbulence in regular polymers. Finally, we will see that non-shear banding wormlike micelles also exhibit elastic instabilities and turbulence, providing a way to explore a large range of elasticity numbers.

Effect of thermal and mechanical rejuvenation on the rheological behavior of chocolate

Chocolate is known to undergo solid–liquid transition upon an increase in temperature as well as under application of deformation field. Upon sudden reduction in temperature from a molten state (or thermal rejuvenation), the rheological properties of chocolate evolve as a function of time under isothermal conditions, a behavior reminiscent of physical aging in polymeric glasses. Then again, subsequent to cessation of shear flow (or mechanical rejuvenation), chocolate shows temporal evolution of the rheological properties, a behavior similar to physical aging in soft glassy materials. In this work, we evaluate three rheological properties—dynamic moduli, relaxation time spectrum, and characteristic relaxation time of chocolate—and compare their evolution after thermal as well as mechanical rejuvenation. We observe that the evolution of the rheological properties subsequent to mechanical rejuvenation is distinctly different from that of thermal rejuvenation, wherein the evolution is more gradual in the former case. On the one hand, this work provides unique insights into how shear affects the rheological behavior of chocolate. On the other hand, this work clearly suggests that chocolate explores different sections of the energy landscape after mechanical rejuvenation compared to that of thermal rejuvenation.

Shear-banding fluid(s) under time-dependent shear flows. Part II: A test of the Moorcroft–Fielding criteria

Complex systems often exhibit shear banding—the coexistence of two different states characterized by their internal structuring and local shear rates. For some of them, the heterogeneous flow corresponds to the final steady-state response, while for others, shear banding can only be transient, the banding structure healing back to a homogeneous flow in the ultimate steady state after long-lived periods. In order to explain the diversity of observations, Moorcroft and Fielding have established general criteria for the onset of banding in time-dependent flows of complex systems, ranging from polymeric fluids to soft glassy materials [Moorcroft et al., Phys. Rev. Lett. 110, 086001 (2013)]. The proposed criteria are based on the time evolution of the bulk rheological response function of the system to a given time-dependent flow protocol and are associated with a specific signature in the mechanical response. In this contribution, we test the validity of these criteria in the case of two common time-dependent flow protocols: a step stress and a shear startup. Two types of fluids are examined. On the one hand, a wormlike micelles system exhibiting steady shear banding is studied experimentally, using rheometry coupled with direct visualizations and particle image velocimetry. On the other hand, we analyze previous literature on yield stress fluids exhibiting transient shear banding. Under conditions of a constant imposed stress, for both types of fluids, the onset of banding arises in a time window compatible with the Moorcroft–Fielding criterion. However, the mechanical signature, i.e., the shape of the bulk mechanical signal as a function of time, is not the one expected within some of the specific models with which the general Moorcroft–Fielding criteria were tested numerically. Under shear startup, both types of fluids behave differently. The criterion holds for yield stress fluids, the onset of banding arising just after the stress overshoot, as expected. On the contrary, for wormlike micelles, the window of instability is delayed, even if the overshoot clearly plays a crucial role in the nucleation of shear-induced structures. Regardless of the flow protocol or the system, wall slip seems to go hand in hand with banding indicating that it is a key ingredient to take into account.

Elastic and Dynamic Heterogeneity in Aging Alginate Gels.

Anomalous aging in soft glassy materials has generated a great deal of interest because of some intriguing features of the underlying relaxation process, including the emergence of “ultra-long-range” dynamical correlations. An intriguing possibility is that such a huge correlation length is reflected in detectable ensemble fluctuations of the macroscopic material properties. We tackle this issue by performing replicated mechanical and dynamic light scattering (DLS) experiments on alginate gels, which recently emerged as a good model-system of anomalous aging. Here we show that some of the monitored quantities display wide variability, including large fluctuations in the stress relaxation and the occasional presence of two-step decay in the DLS decorrelation functions. By quantifying elastic fluctuation through the standard deviation of the elastic modulus and dynamic heterogeneities through the dynamic susceptibility, we find that both quantities do increase with the gel age over a comparable range. Our results suggest that large elastic fluctuations are closely related to ultra-long-range dynamical correlation, and therefore may be a general feature of anomalous aging in gels.

Modeling, measurements and validation of magnetic field dependent flow behavior of magnetorheological fluids; static and dynamic yield stress

The issue which is always debated in the magnetorheological (MR) fluid is the dynamic and static yield stress values obtained from a flow curve. To evaluate these, a suitable constitutive equations are required. The aim of this article is to provide suitable rheological models for the prediction of static and dynamic yield stress from a single flow curve under wide shear rate and magnetic field ranges. The proposed model is well suited for isotropic and anisotropic particle-based MR fluids. The parameters, like yield stress, critical shear rate and viscosity at high shear rate obtained from the fit show systematic variation with applied magnetic field strength. These variations in the parameters can be related to the mesostructure formation and its influence on the MR suspension rheological properties. To further confirm the value of the static yield stress derived from the proposed model, the small-strain oscillatory shear flow experiment is carried out and the static yield stress values are obtained. These values agree well with that obtained from the proposed model. Similarly, to evaluate the dynamic yield stress from the flow curve, modified three parameter model applied to soft glassy materials is used. Here, the critical shear rate parameter obtained from the proposed model is used to deduce dynamic yield stress. It is identified that the values obtained are lower than the static yield stress. Furthermore, at low magnetic field, the static and dynamic yield stress follows the relationship defined by the square dependence of the magnetic field strength. The difference between the static and dynamic yield stress values at high field is greater for anisotropic particle based MR fluid than isotropic particle based MR fluid. This reflects the contribution of particle-particle and particle-carrier interaction in the value of the yield stress in anisotropic particle based MR fluid. Thus, the present model provides a clear idea of the dissipation process that occurs in a shear MR fluid.

Non-Newtonian laminar flow in pipes using radius, stress, shear rate or velocity as the independent variable

The flow of a non-Newtonian fluid in a circular pipe is a classic introductory transport phenomena problem, familiar to readers of Robert Byron Bird textbooks. A characteristic of Bird's work was taking the time to explore alternative ways to describe a problem and refine the results into elegant and readable formulas. Inspired by that approach, we compare methods for pipe flow solutions that differ on the independent variable used (radius, stress, shear rate) to obtain flow rate and residence time distributions for generalized Newtonian fluids. We highlight cases where using the shear rate as the independent variable has advantages for analytical and numerical solutions. We describe a method to use velocimetry experimental data coupled with a pressure drop measurement to directly construct a curve of flow rate vs pressure drop without the need of fitting the data to any rheological models. We present a geometrical interpretation of velocity profiles as areas in the stress–shear rate plane and derive analytical solutions for a three-parameter model of soft glassy materials [Caggioni et al., “Variations of the Herschel–Bulkley exponent reflecting contributions of the viscous continuous phase to the shear rate-dependent stress of soft glassy materials,” J. Rheol. 64, 413 (2020)] and a four-parameter model for chocolate melts [H. D. Tscheuschner, “Rheologische eigenschaften von lebensmittelsystemen,” in Rheologie Der Lebensmittel, edited by D. Weipert, H. Tscheuschner, and E. Windhab (Behr's Verlag, Hamburg, 1993), pp. 101–172]. We also compare the speed of various numerical approaches for a fractional viscoelastic model [A. Jaishankar and G. H. McKinley, “A fractional K-BKZ constitutive formulation for describing the nonlinear rheology of multiscale complex fluids,” J. Rheol. 58, 1751 (2014)].

Stress Overshoots in Simple Yield Stress Fluids.

Soft glassy materials such as mayonnaise, wet clays, or dense microgels display a solid-to-liquid transition under external shear. Such a shear-induced transition is often associated with a nonmonotonic stress response in the form of a stress maximum referred to as "stress overshoot." This ubiquitous phenomenon is characterized by the coordinates of the maximum in terms of stress σ_{M} and strain γ_{M} that both increase as weak power laws of the applied shear rate. Here we rationalize such power-law scalings using a continuum model that predicts two different regimes in the limit of low and high applied shear rates. The corresponding exponents are directly linked to the steady-state rheology and are both associated with the nucleation and growth dynamics of a fluidized region. Our work offers a consistent framework for predicting the transient response of soft glassy materials upon startup of shear from the local flow behavior to the global rheological observables.

Continuum modeling of shear startup in soft glassy materials.

Yield stress fluids (YSFs) display a dual nature highlighted by the existence of a critical stress σ_{y} such that YSFs are solid for stresses σ imposed below σ_{y}, whereas they flow like liquids for σ>σ_{y}. Under an applied shear rate γ[over ̇], the solid-to-liquid transition is associated with a complex spatiotemporal scenario that depends on the microscopic details of the system, on the boundary conditions, and on the system size. Still, the general phenomenology reported in the literature boils down to a simple sequence that can be divided into a short-time response characterized by the so-called "stress overshoot," followed by stress relaxation towards a steady state. Such relaxation can be either (1) long-lasting, which usually involves the growth of a shear band that can be only transient or that may persist at steady state or (2) abrupt, in which case the solid-to-liquid transition resembles the failure of a brittle material, involving avalanches. In the present paper, we use a continuum model based on a spatially resolved fluidity approach to rationalize the complete scenario associated with the shear-induced yielding of YSFs. A key feature of our model is to provide a scaling for the coordinates of the stress overshoot, i.e., stress σ_{M} and strain γ_{M} as a function of γ[over ̇], which shows good agreement with experimental and numerical data extracted from the literature. Moreover, our approach shows that the power-law scaling σ_{M}(γ[over ̇]) is intimately linked to the growth dynamics of a fluidized boundary layer in the vicinity of the moving boundary. Yet such scaling is independent of the fate of that layer, and of the long-term behavior of the YSF, i.e., whether the steady-state flow profile is homogeneous or shear-banded. Finally, when including the presence of "long-range" correlations, we show that our model displays a ductile to brittle transition, i.e., the stress overshoot reduces into a sharp stress drop associated with avalanches, which impacts the scaling σ_{M}(γ[over ̇]). This generalized model nicely captures subtle avalanche-like features of the transient shear banding dynamics reported in experiments. Our work offers a unified picture of shear-induced yielding in YSFs, whose complex spatiotemporal dynamics are deeply connected to nonlocal effects.

Rheological behaviour of attractive emulsions differing in droplet-droplet interaction strength

HypothesisWe hypothesise that interaction strength between oil droplets determine the rheological properties of oil-in-water (O/W) emulsions by simultaneous formation and break-up of bonds between droplets. Using small (SAOS) and large (LAOS) amplitude oscillatory shear measurements, we aim to distinguish different classes of emulsions based on the specific microstructural evolution of the emulsions. ExperimentsConcentrated O/W emulsions differing in droplet-droplet interaction strength were obtained. Different interaction strength was obtained using different types of interactions; (a) electrostatic attraction, (b) salt bridging, or (c) crosslinking. FindingsIn line with our hypothesis, different rheological events in emulsions depend on the droplet-droplet interaction strength. Strong interactions lead to monotonous yielding, and droplets undergo jamming or densification to provide strain hardening and gel-like behaviour. Emulsions with weak interactions exhibit two-step yielding (SAOS) and continuous yielding in LAOS; indicating a soft-glassy material. In emulsions above maximum packing, and with weak interactions the rheology is controlled by cluster/cage breaking, and transient formation of new clusters. For medium-strength interactions, two-step yielding was reduced, and apparent stain-hardening occurred. The probability of two distinct time scales of yielding is hindered by stronger interactions and jamming. Overall, in concentrated emulsions, yielding is determined by network rupture and reformation, cluster rearrangement and -breaking, which in turn is influenced by interaction type and strength. We present a more differentiated categorisation of emulsions based on interaction strength.

Analogies between growing dense active matter and soft driven glasses

We develop a minimal model to describe growing dense active matter such as biological tissues, bacterial colonies and biofilms, that are driven by a competition between particle division and steric repulsion. We provide a detailed numerical analysis of collective and single particle dynamics. We show that the microscopic dynamics can be understood as the superposition of an affine radial component due to the global growth, and of a more complex non-affine component which displays features typical of driven soft glassy materials, such as aging, compressed exponential decay of time correlation functions, and a crossover from superdiffusive behaviour at short scales to subdiffusive behaviour at larger scales. This analogy emerges because particle division at the microscale leads to a global expansion which then plays a role analogous to shear flow in soft driven glasses. We conclude that growing dense active matter and sheared dense suspensions can generically be described by the same underlying physics.