Forces between solid surfaces in aqueous electrolyte solutions

Alexander M. Smith, Michal Borkovec, Gregor Trefalt

This review addresses experimental findings obtained with direct force measurements between two similar or dissimilar solid surfaces in aqueous electrolyte solutions. Interpretation of these measurements is mainly put forward in terms of the classical theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO). This theory invokes a superposition of attractive van der Waals forces and repulsive double layer forces. DLVO theory is shown to be extremely reliable, even in the case of multivalent ions. However, such a description is only successful, when appropriate surface charge densities, charge regulation characteristics, and ion pairing or complexation equilibria in solution are considered. Deviations from DLVO theory only manifest themselves at distances of typically below few nm. More long-ranged non-DLVO forces can be observed in some situations, particularly, in concentrated electrolyte solutions, in the presence of strongly adsorbed layers, or for hydrophobic surfaces. The latter forces probably originate from patch-charge surface heterogeneities, which can be induced by ion-ion correlation effects, charge fluctuations, or other types of surface heterogeneities.

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A practical review on the measurement tools for cellular adhesion force

Rita Ungai-Salánki, Beatrix Peter, Tamás Gerecsei, Norbert Orgovan, Robert Horvath, Bálint Szabó

Cell–cell and cell–matrix adhesions are fundamental in all multicellular organisms. They play a key role in cellular growth, differentiation, pattern formation and migration. Cell-cell adhesion is substantial in the immune response, pathogen-host interactions, and tumor development. The success of tissue engineering and stem cell implantations strongly depends on the fine control of live cell adhesion on the surface of natural or biomimetic scaffolds. Therefore, the quantitative and precise measurement of the adhesion strength of living cells is critical, not only in basic research but in modern technologies, too. Several techniques have been developed or are under development to quantify cell adhesion. All of them have their pros and cons, which has to be carefully considered before the experiments and interpretation of the recorded data. Current review provides a guide to choose the appropriate technique to answer a specific biological question or to complete a biomedical test by measuring cell adhesion.

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Microrheology, advances in methods and insights

Qiuyang Xia, Huining Xiao, Yuanfeng Pan, Lidong Wang

Microrheology is an emerging technique that probes mechanical response of soft material at micro-scale. Generally, microrheology technique can be divided into active and passive versions. For active microrheology, a user-controlled force, e.g. magnetic force, electrostatic force, optical tweezers etc., is applied to embedded particle in medium of interest, and the particle motion under this force is tracked. For passive microrheology, the embedded particles only move due to thermal fluctuations in the medium, i.e. Brownian motion, and their trajectories are tracked and analyzed. After Mason's seminal paper that developed reliable theory to calculate the relation between viscoelastic property and Brownian motion of embedding particles, corresponding methods and equipments to track particle Brownian motion both in laboratory and commercially available, together with software for data analysis. During last two decades, extensive efforts have been paid to improve both the experiment techniques and data analysis methods, especially about how to link consequential particle positions into trajectories. Also, some insights have been revealed in soft matter system using this technique. In this review paper we attempt to go through the advances in experiment techniques and data analysis methods developed in last ten years along with some recent results obtained from these methods.

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Recent advances in studying single bacteria and biofilm mechanics

Even C, Marlière C, Ghigo JM, Allain JM, Marcellan A, Raspaud E

Bacterial biofilms correspond to surface-associated bacterial communities embedded in hydrogel-like matrix, in which high cell density, reduced diffusion and physico-chemical heterogeneity play a protective role and induce novel behaviors. In this review, we present recent advances on the understanding of how bacterial mechanical properties, from single cell to high-cell density community, determine biofilm tri-dimensional growth and eventual dispersion and we attempt to draw a parallel between these properties and the mechanical properties of other well-studied hydrogels and living systems.

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Relevance of interfacial viscoelasticity in stability and conformation of biomolecular organizates at air/fluid interface

M. Steffi Antony, Jaganathan Maheshkumar, Aruna Dhathathreyan

Soft materials are complex macromolecular systems often exhibiting perplexing non-Newtonian viscoelastic properties, especially when the macromolecules are entangled, crowded or cross-linked. These materials are ubiquitous in biology, food and pharma industry and have several applications in biotechnology and in the field of biosensors. Based on the length scales, topologies, flexibility and concentration, the systems behave both as liquids (viscous) and solids (elastic). Particularly, for proteins and protein-lipid systems, viscoelasticity is an important parameter because it often relates directly to stability and thermodynamic interactions of the pure biological components as well as their mixtures. Despite the large body of work that is available in solution macro-rheometry, there remain still a number of issues that need to be addressed in dealing with proteins at air/fluid interfaces and with protein-polymer or protein-lipid interfaces that often exhibit very low interfacial viscosity values.
Considering the important applications that they have in biopharmaceutical, biotechnological and nutraceutical industries, there is a need for developing methods that meet the following three specific issues: small volume; large dynamic range of shear rates; and interfacial properties of different biomolecules. Further, the techniques that are developed should include Newtonian, shear thinning and yielding properties, which are representative of the different solution behaviors typically encountered. The review presented here is a comprehensive account of the rheological properties of different biomolecules at air/fluid and solid/fluid interfaces. It addresses the usefulness of ‘viscoelasticity’ of the systems at the interfaces analyzed at the molecular level that can be correlated with the microscopic material properties and touches upon some recent techniques in microrheology that are being used to measure the unusually low viscosity values sensitively.

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Microcapsule mechanics: From stability to function

Martin P. Neubauer, Melanie Poehlmann, Andreas Fery

Microcapsules are reviewed with special emphasis on the relevance of controlled mechanical properties for functional aspects. At first, assembly strategies are presented that allow control over the decisive geometrical parameters, diameter and wall thickness, which both influence the capsule's mechanical performance. As one of the most powerful approaches the layer-by-layer technique is identified. Subsequently, ensemble and, in particular, single-capsule deformation techniques are discussed. The latter generally provide more in-depth information and cover the complete range of applicable forces from smaller than pN to N. In a theory chapter, we illustrate the physics of capsule deformation. The main focus is on thin shell theory, which provides a useful approximation for many deformation scenarios. Finally, we give an overview of applications and future perspectives where the specific design of mechanical properties turns microcapsules into (multi-)functional devices, enriching especially life sciences and material sciences.

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Recent developments in the field of bending rigidity measurements on membranes

Rumiana Dimova

This review gives a brief overview of experimental approaches used to assess the bending rigidity of membranes. Emphasis is placed on techniques based on the use of giant unilamellar vesicles. We summarize the effect on the bending rigidity of membranes as a function of membrane composition, presence of various inclusions in the bilayer and molecules and ions in the bathing solutions. Examples for the impact of temperature, cholesterol, some peptides and proteins, sugars and salts are provided and the literature data discussed critically. Future directions, open questions and possible developments in this research field are also included.

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Particle laden fluid interfaces: Dynamics and Interfacial rheology

Alma J. Mendoza, Eduardo Guzman, Fernando Martinez-Pedrero, Herman Ritacco, Ramon G. Rubio, Francisco Ortega, Victor M. Starov, Reinhard Miller
We review the dynamics of particle laden interfaces, both particle monolayers and particle + surfactant monolayers. We also discuss the use of the Brownian motion of microparticles trapped at fluid interfaces for measuring the shear rheology of surfactant and polymer monolayers. We describe the basic concepts of interfacial rheology and the different experimental methods for measuring both dilational and shear surface complex moduli over a broad range of frequencies, with emphasis in the micro-rheology methods. In the case of particles trapped at interfaces the calculation of the diffusion coefficient from the Brownian trajectories of the particles is calculated as a function of particle surface concentration. We describe in detail the calculation in the case of subdiffusive particle dynamics. A comprehensive review of dilational and shear rheology of particle monolayers and particle + surfactant monolayers is presented. Finally the advantages and current open problems of the use of the Brownian motion of microparticles for calculating the shear complex modulus of monolayers are described in detail.

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