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Colloidal gels are a prototypical example of a heterogeneous network solid whose complex properties are governed by thermally-activated dynamics. In this Letter we experimentally establish the connection between the intermittent dynamics of individual particles and their local connectivity. We interpret our experiments with a model that describes single-particle dynamics based on highly cooperative thermal debonding. The model, in quantitative agreement with experiments, provides a microscopic picture for the structural origin of dynamical heterogeneity in colloidal gels and sheds new light on the link between structure and the complex mechanics of these heterogeneous solids.Attractive interactions can drive a dilute colloidal suspension towards a solid state formed by a samplespanning and mechanically-rigid particle network [1,2]. These colloidal gels are non-equilibrium solids, kinetically arrested en route to their equilibrium state of solidliquid coexistence [3]. Such particle gels are characterized by strong heterogeneity in their local connectivity, mesoscopic structure and their dynamics and mechanics [4][5][6][7]. The microstructure and internal dynamics of colloidal gels can be directly observed with microscopy techniques at the single-particle level. As a consequence, it forms an interesting testing ground to explore the complex and length-scale dependent mechanics of heterogeneous solids. Colloidal gels derive their mechanical rigidity from physically bonded gel strands and nodes that form a percolating elastic network. The linear elasticity of gels is governed by the mechanics of the network architecture and its thermal fluctuations [8,9]. By contrast, the gradual aging of gels to a denser state [1,10] and their non-linear response to applied stresses [11,12], is governed by events occuring at the the much smaller length scale of individual particles. Since the bonds between the particles are typically weak, single particles can debond from strands in the gel by thermally-activated bond breaking [13]. On longer time scales, this result in the gradual restructuration of the gel network, causing it to coarsen, age and relax internal stresses that are built up during gelation [14]. Moreover, thermal-activation at the single particle level plays a crucial role in processes of fatigue that preempt stress-induced failure of the gel network [11]. To date, quantitative descriptions of these thermally-activated phenomena have relied on mean-field approximations [13]. Yet, the inhomogeneity in local coordination that is intrinsic to gels, must play a large role in the intermittent debonding dynamics that are at the origin of this complex non-linear behavior. As a result, linking the structure of colloidal gels to their non-linear mechanics has remained challenging, in particular as the relationship between local connectivity and thermallyactivated dynamics of single particles is not clearly established.In this letter we explore the connection between the local connectivity and intermittent bonding-debonding dy...

Phytophthora infestans , causal agent of late blight in potato and tomato, remains challenging to control. Unravelling its biomechanics of host invasion, and its response to mechanical and chemical stress, could provide new handles to combat this devastating pathogen. Here we introduce two fluorescent molecular sensors, CWP-BDP and NR12S, that reveal the micromechanical response of the cell wall-plasma membrane continuum in P. infestans during invasive growth and upon chemical treatment. When visualized by live-cell imaging, CWP-BDP reports changes in cell wall (CW) porosity while NR12S reports variations in chemical polarity and lipid order in the plasma membrane (PM). During invasive growth, mechanical interactions between the pathogen and a surface reveal clear and localized changes in the structure of the CW. Moreover, the molecular sensors can reveal the effect of chemical treatment to CW and/or PM, thereby revealing the site-of-action of crop protection agents. This mechano-chemical imaging strategy resolves, non-invasively and with high spatio-temporal resolution, how the CW-PM continuum adapts and responds to abiotic stress, and provides information on the dynamics and location of cellular stress responses for which, to date, no other methods are available.

Filamentous plant pathogens apply mechanical forces to pierce their hosts surface and penetrate its tissues. Devastating Phytophthora pathogens harness a specialized form of invasive tip growth to slice through the plant surface, wielding their hypha as a microscopic knife. Slicing requires a sharp hyphal tip that is not blunted at the site of the mechanical interaction. How tip shape is controlled, however, is unknown. We uncover an actin-based mechanostat in Phytophthora infestans that controls tip sharpness during penetration. Mechanical stimulation of the hypha leads to the emergence of an aster-like actin configuration, which shows fast, local, and quantitative feedback to the local stress. We evidence that this functions as an adaptive mechanical scaffold that sharpens the invasive weapon and prevents it from blunting. The hyphal tip mechanostat enables the efficient conversion of turgor into localized invasive pressures that are required to achieve host penetration.

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