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Register a new userSticky Surfaces: Sphere-Sphere Adhesion Dynamics
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We present a multi-scale model to study the attachment of spherical particles with a rigid core, coated with binding ligands and in equilibrium with the surrounding, quiescent fluid medium. This class of fluid-immersed adhesion is widespread in many natural and engineering settings. Our theory highlights how the micro-scale binding kinetics of these ligands, as well as the attractive / repulsive surface potential in an ionic medium effects the eventual macro-scale size distribution of the particle aggregates (flocs). The results suggest that the presence of elastic ligands on the particle surface allow large floc aggregates by inducing efficient inter-floc collisions (i.e., a large, non-zero collision factor). Strong electrolytic composition of the surrounding fluid favors large floc formation as well.
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The near-surface swimming patterns of bacteria are strongly determined by the hydrodynamic interactions between bacteria and the surface, which trap bacteria in smooth circular trajectories that lead to inefficient surface exploration. Here, we show by combining experiments and a data-driven mathematical model that surface exploration of enterohemorrhagic Escherichia coli (EHEC) -- a pathogenic strain of E. coli causing serious illnesses such as bloody diarrhea -- results from a complex interplay between motility and transient surface adhesion events. These events allow EHEC to break the smooth circular trajectories and regulate their transport properties by the use stop-adhesion events that lead to a characteristic intermittent motion on surfaces. We find that the experimentally measured frequency of stop-adhesion events in EHEC is located at the value predicted by the developed mathematical model that maximizes bacterial surface diffusivity. We indicate that these results and the developed model apply to other bacterial strains on different surfaces, which suggests that swimming bacteria use transient adhesion to regulate surface motion.
Cytoskeletal motor proteins are involved in major intracellular transport processes which are vital for maintaining appropriate cellular function. The motor exhibits distinct states of motility: active motion along filaments, and effectively stationary phase in which it detaches from the filaments and performs passive diffusion in the vicinity of the detachment point due to cytoplasmic crowding. The transition rates between motion and pause phases are asymmetric in general, and considerably affected by changes in environmental conditions which influences the efficiency of cargo delivery to specific targets. By considering the motion of molecular motor on a single filament as well as a dynamic filamentous network, we present an analytical model for the dynamics of self-propelled particles which undergo frequent pause phases. The interplay between motor processivity, structural properties of filamentous network, and transition rates between the two states of motility drastically changes the dynamics: multiple transitions between different types of anomalous diffusive dynamics occur and the crossover time to the asymptotic diffusive or ballistic motion varies by several orders of magnitude. We map out the phase diagrams in the space of transition rates, and address the role of initial conditions of motion on the resulting dynamics.
A relation $mathcal{M}_{mathrm{SHS}tomathrm{LJ}}$ between the set of non-isomorphic sticky hard sphere clusters $mathcal{M}_mathrm{SHS}$ and the sets of local energy minima $mathcal{M}_{LJ}$ of the $(m,n)$-Lennard-Jones potential $V^mathrm{LJ}_{mn}(r) = frac{varepsilon}{n-m} [ m r^{-n} - n r^{-m} ]$ is established. The number of nonisomorphic stable clusters depends strongly and nontrivially on both $m$ and $n$, and increases exponentially with increasing cluster size $N$ for $N gtrsim 10$. While the map from $mathcal{M}_mathrm{SHS}to mathcal{M}_{mathrm{SHS}tomathrm{LJ}}$ is non-injective and non-surjective, the number of Lennard-Jones structures missing from the map is relatively small for cluster sizes up to $N=13$, and most of the missing structures correspond to energetically unfavourable minima even for fairly low $(m,n)$. Furthermore, even the softest Lennard-Jones potential predicts that the coordination of 13 spheres around a central sphere is problematic (the Gregory-Newton problem). A more realistic extended Lennard-Jones potential chosen from coupled-cluster calculations for a rare gas dimer leads to a substantial increase in the number of nonisomorphic clusters, even though the potential curve is very similar to a (6,12)-Lennard-Jones potential.
The collective dynamics of liquid Gallium close to the melting point has been studied using Inelastic X-ray Scattering to probe lengthscales smaller than the size of the first coordination shell. %(momentum transfers, $Q$, $>$15 nm$^{-1}$). Although the structural properties of this partially covalent liquid strongly deviate from a simple hard-sphere model, the dynamics, as reflected in the quasi-elastic scattering, are beautifully described within the framework of the extended heat mode approximation of Enskogs kinetic theory, analytically derived for a hard spheres system. The present work demonstrates the applicability of Enskogs theory to non hard- sphere and non simple liquids.
Respiration in bacteria involves a sequence of energetically-coupled electron and proton transfers creating an electrochemical gradient of protons (a proton-motive force) across the inner bacterial membrane. With a simple kinetic model we analyze a redox loop mechanism of proton-motive force generation mediated by a molecular shuttle diffusing inside the membrane. This model, which includes six electron-binding and two proton-binding sites, reflects the main features of nitrate respiration in E. coli bacteria. We describe the time evolution of the proton translocation process. We find that the electron-proton electrostatic coupling on the shuttle plays a significant role in the process of energy conversion between electron and proton components. We determine the conditions where the redox loop mechanism is able to translocate protons against the transmembrane voltage gradient above 200 mV with a thermodynamic efficiency of about 37%, in the physiologically important range of temperatures from 250 to 350 K.
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