Soft Condensed Matter
Investigating the normal and tangential peeling behaviour of gecko spatulae using a coupled adhesion-friction model (1901.11505v1)
Saipraneeth Gouravaraju, Roger A. Sauer, Sachin Singh Gautam
The present work investigates the normal and tangential peeling behaviour of a gecko spatula computationally using a coupled adhesion-friction model. The objective is to explain the strong attachment and easy detachment behaviour of the spatulae as well as to understand the principles behind their optimum design. Using a computational model, it is shown that the "frictional adhesion" behaviour, until now only observed from seta to toe levels, is also present at the spatula level. The model also shows that there is an optimum range of spatula pad thickness for which, irrespective of the peeling angle, the spatula detaches at a constant angle known as critical detachment angle. It is shown that the spatula readily detaches from the substrate by changing its shaft angle and then peeling vertically like a tape. Additionally, it is found that friction increases the attachment forces, while the detachment forces are fairly unaffected. Since the present computational model is not limited by the geometrical, kinematical, and material restrictions of theoretical models, it can be employed to study and analyse the adhesion behaviour of many similar biological adhesive systems.
Andrea Ninarello, Jérôme J. Crassous, Divya Paloli, Fabrizio Camerin, Nicoletta Gnan, Lorenzo Rovigatti, Peter Schurtenberger, Emanuela Zaccarelli
Thermoresponsive microgels are soft colloids that find widespread use as model systems for soft matter physics. Their complex internal architecture, made of a disordered and heterogenous polymer network, has been so far a major challenge for computer simulations. In this work we introduce an advanced coarse-grained model of microgels whose structural properties are in quantitative agreement with results obtained with small-angle X-ray scattering experiments across a wide range of temperatures, encompassing the volume phase transition. These results bridge the gap between experiments and simulations of individual microgel particles, paving the way to theoretically address open questions about their bulk properties with unprecedented microscopic resolution.
Condensation of nucleoid in Escherichia coli cell as a result of prolonged starvation (1901.11322v1)
N. G. Loiko, Ya. A. Danilova, A. V. Moiseenko, E. V. Demkina, Kovalenko V. V, ., K. B. Tereshkina, G. I. El-Registan, O. S. Sokolova, Yu. F. Krupyanskii
Electron microscopy and X-ray diffraction studies of dormant E. coli cells revealed several forms of nucleoid condensation: quasi - nanocrystalline, quasi -liquid crystalline (or spore-like) and folded nucleosome - like structure. Of particular interest is the third type of structure since it was described here for the first time: the folded nucleosome-like. Such a structure has no relation to the toroidal DNA organization, which are the intermediate form in the formation of the quasi - nanocrystalline structure. Results observed here shed a new light both on the phenomenon of nucleoid condensation in prokaryotic cells and on the general problem of developing a response to stress. It was found out that there is no single mechanism for nucleoid condensation in the population of a dormant cell; diversity in their number, shape and packing has been seen. According to the recognized concept of a bacterial population as a multicellular organism, its heterogeneity allows to respond flexibly to the environmental changes and to survive in stressful situations. That is the reason why we observed at least three types of nucleoid condensation in dormant E. coli cells. Heterogeneity of dormant cells increases the ability of the whole population to survive under various stress conditions. For better understanding of the nucleoid condensation mechanism, it is necessary to study the reverse transition of the nucleoid in bacteria from the dormant to the functional state.
Driven and undriven states of multicomponent granular gases of inelastic and rough hard disks or spheres (1901.11307v1)
Alberto Megías, Andrés Santos
Starting from a recent derivation of the energy production rates in terms of the number of translational and rotational degrees of freedom, a comparative study on different granular temperatures in gas mixtures of inelastic and rough disks or spheres is carried out. Both the homogeneous freely cooling state and the state driven by a stochastic thermostat are considered. It is found that the relaxation number of collisions per particle is generally smaller for disks than for spheres, the mean angular velocity relaxing more rapidly than the temperature ratios. In the asymptotic regime of the undriven system, the rotational-translational nonequipartition is stronger in disks than for spheres, while it is hardly dependent on the class of particles in the driven system. On the other hand, the degree of component-component nonequipartition is higher for spheres than for disks, both for driven and undriven systems. A study of the mimicry effect (whereby a multicomponent gas mimics the rotational-translational temperature ratio of a monocomponent gas) is also undertaken.
Energy-optimal strokes for multi-link microswimmers: Purcell's loops and Taylor's waves reconciled (1801.04687v3)
François Alouges, Antonio DeSimone, Laetitia Giraldi, Yizhar Or, Oren Wiezel
Micron-scale swimmers move in the realm of negligible inertia, dominated by viscous drag forces. In this paper, we formulate the leading-order dynamics of a slender multi-link (N-link) microswimmer assuming small-amplitude undulations about its straight configuration. The energy-optimal stroke to achieve a given prescribed displacement in a given time period is obtained as the largest eigenvalue solution of a constrained optimal control problem. Remarkably, the optimal stroke is an ellipse lying within a two-dimensional plane in the (N-1)-dimensional space of joint angles, where N can be arbitrarily large. For large N, the optimal stroke is a traveling wave of bending, modulo edge effects. If the number of shape variables is small, we can consider the same problem when the prescribed displacement in one time period is large, and not attainable with small variations of the joint angles. The fully nonlinear optimal control problem is solved numerically for the cases N=3 (Purcell's three-link swimmer) and N=5 showing that, as the prescribed displacement becomes small, the optimal solutions obtained using the small-amplitude assumption are recovered. We also show that, when the prescribed displacements become large, the picture is different. For N=3 we recover the non-convex planar loops already known from previous studies. For N=5 we obtain non-planar loops, raising the question of characterizing the geometry of complex high-dimensional loops.
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