How Things Get Stuck: Kinetics, Elastohydrodynamics, and Soft Adhesion (original) (raw)

Sticky Surfaces: Sphere-Sphere Adhesion Dynamics

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.

Effect of hydrodynamic interaction on polymeric tethers

Physical Review E, 2010

Weak bonds are ubiquitous in biological structures. They often act as adhesive contacts within an extended structure, for example, the internal bonds in a folded protein or a DNA/RNA loop. They also act as linkers between two structures, for example, a protein grafted in a cell membrane or a protein linking the cell membranes of two neighboring cells. Typically, the breakage of a bond depends on the strength of the binding potential and viscosity of the medium. But when extended structures couple to the bond, as in the above examples, the dynamics of the structure also has to be considered in order to understand the bond breakage phenomenon. Here we consider a generic model, a stretched polymer ͑an extended structure͒ tethered to a soft bond and study how the dynamics of the polymer, in addition to thermal noise, influences bond breakage. We also explore how the hydrodynamic interaction due to the fluid medium, which couples the distant parts of the polymer, change the bond breakage rate. We find that hydrodynamic interaction enhances the breakage rate and also makes the motion of the unstable collective mode of the polymer more coherent.

Wetting and phase separation in soft adhesion

Proceedings of the National Academy of Sciences, 2015

Significance Modern contact mechanics was originally developed to describe adhesion to relatively stiff materials like rubber, but much softer sticky materials are ubiquitous in biology, medicine, engineering, and everyday consumer products. By studying adhesive contact between compliant gels and rigid objects, we demonstrate that soft materials adhere very differently than their stiffer counterparts. We find that the structure in the region of contact is governed by the same physics that sets the geometry of liquid droplets, even though the material is solid. Furthermore, adhesion can cause the local composition of a soft material to change, thus coupling to its thermodynamic properties. These findings may substantially change our understanding of the mechanics of soft contact.

Adhesion of soft objects on wet substrates

We study the dynamics of contact of a soft object (rubber bead, soft shell, vesicles, living cells) on a wet substrate by removal of the intercalated liquid film. The profiles of the contact zone are observed by reflection interference contrast microscopy. The adhesion forces (either hydrophobic, electrostatic or specific) are measured by micropipettes, flow cells or 'microkarcher' techniques. For vesicles, the adhesion induces a tension of the membrane, which relaxes by the formation of transient macroscopic pores. We study the dynamics of opening and closing of pores.

Adhesion induced by mobile binders: Dynamics

Proceedings of the National Academy of Sciences, 2002

We consider a vesicle bilayer loaded with molecules that can bind (upon contact) with a solid surface, following the classical model of Bell, Dembo, and Bongrand. We are interested in situations where the contact area varies with time: we assume that binders can then migrate via diffusion. The resulting dissipation and lag create a retarded force on the contact line, which could be significant in squeezing or rolling experiments. However, there are two cases where we expect the lag force to be ineffective: (i) separation by shrinking of an adhesive patch (where the Evans ''tear out'' process turns out to be less costly) and (ii) spontaneous growth of a patch from a point contact. In this last case, the lag force is weak, and we give detailed predictions for the growth laws.

Non-equilibrium behavior of sticky colloidal particles: beads, clusters and gels

The European Physical Journal E, 2005

To understand the non-equilibrium behavior of colloidal particles with short-range attraction, we studied salt-induced aggregation of lysozyme. Optical microscopy revealed four regimes: bicontinuous texture, 'beads', large aggregates, and transient gelation. The interaction of a metastable liquid-liquid binodal and an ergodic to non-ergodic transition boundary inside the equilibrium crystallization region can explain our findings.

Statics and Dynamics of Polymer-Wrapped Colloids †

The Journal of Physical Chemistry B, 2005

We study the complex between a colloidal particle and a semiflexible polymer chain that "wraps" around it. Via molecular dynamics simulation we investigate statistical and dynamical properties of this system. First we establish the dependence of wrapped chain length on absorption energy and chain persistence length and obtain the distribution of wrapped-sphere positions. Then we study the length and position distributions of thermally excited loop defects. Finally we consider the repositioning dynamics of the colloid, focusing on the case where the chain stays wrapped onto the complex. Specifically we determine the mean square displacement of the central monomer of the wrapped chain and the resulting diffusion coefficient of the chain as a function of its persistence length, absorption energy, chain length, and size of the sphere. We argue that both statics and dynamics of these complexes can be mainly understood by energetic arguments, whereas entropic contributions from the chain configurations play only a minor role. † Part of the special issue "Irwin Oppenheim Festschrift". ‡ Universidad Veracruzana.

Chain Stiffness and Attachment-Dependent Attraction between Polyelectrolyte-Grafted Colloids

2010

We report here the effects of chain stiffness and surface attachment on the effective interactions between polyelectrolyte-grafted colloidal particles in monovalent salt obtained using Monte Carlo simulations. Our approach involves computation of the distance-dependent potential of mean force between two polyelectrolytegrafted colloidal particles treated at a coarse-grained resolution. Two chain stiffnesses, flexible and stiff, and two surface attachment modes, free and constrained, at low grafting densities are examined. PMF calculations across a range of surface and polyelectrolyte charge allows us to map out the strength and extent of the attractive and repulsive regime in the two-dimensional charge space. We observe striking differences in the effects of chain stiffness between the two modes of attachment. When the chains are freely attached, the stiff-chains colloids exhibit a marginal reduction in the attraction compared to their flexible-chain counterparts. In contrast, when the chains are attached in a constrained manner, the colloids with stiff chains exhibit a significantly stronger attraction and a broader attractive regime compared to flexible-chain colloids. These differences in the effects of stiffness between the two attachment modes are explained in terms of differences in the energetic and entropic forces balancing adsorption of chains at their own surface versus chain extension to mediate bridging interactions across two particles. Our results thus underscore the importance of surface attachment of chains and its proper accounting in computational and experimental studies and suggests the mode of chain attachment as an additional control parameter for modulating intercolloid interactions for applications such as stabilization of colloidal systems and bottom-up self-assembly of nanostructures.

Interface Stabilization in Adhesion Caused by Elastohydrodynamic Deformation

Physical Review Letters

Interfacial instabilities are common phenomena observed during adhesion measurements involving viscoelastic polymers or fluids. Typical probe-tack adhesion measurements with soft adhesives are conducted with rigid probes. However, in many settings, such as for medical applications, adhesives make and break contact from soft surfaces such as skin. Here we study how detachment from soft probes alters the debonding mechanism of a model viscoelastic polymer film. We demonstrate that detachment from a soft probe suppresses Saffman-Taylor instabilities commonly encountered in adhesion. We suggest the mechanism for interface stabilization is elastohydrodynamic deformation of the probe and propose a scaling for the onset of stabilization.