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Editorial Overview – Surface Forces
Current Opinion in Colloid & Interface Science 27 (2017) A1–A2
Contents lists available at ScienceDirect
Current Opinion in Colloid & Interface Science journal homepage: www.elsevier.com/locate/cocis
Editorial Overview – Surface Forces William Ducker and Per Claesson
WILLIAM DUCKER William Ducker was born and educated in Australia, with a B.Sc. and Ph.D. from the Australian National University. He worked as a Post-Doctoral researcher at the University of California in Santa Barbara, a lecturer at the University of Otago and a Professor at the University of Melbourne. He is now at Virginia Tech in the USA. His research interests include: Surface Forces, Lubrication, Surface Organization, Atomic Force Microscopy, Stability of Colloids, Surfactants, Interface Thermal Conductance, and Bacterial Adhesion.
PER CLAESSON Per Claesson is professor in Surface Chemistry at the Royal Institute of Technology, KTH, Stockholm. He started his scientific career as PhD-student at KTH and during that time he enjoyed learning about surface forces during a one-year visit to The Australian National University, being exposed to the knowledge of Pashley, Israelachvili, Horn, Christenson and Ninham. Studies of surface forces have remained a central theme since that time. He has led several large collaborations involving academic and industrial partners, such as the competence centre “Surfactants Based on Natural Products” and the program on “Microstructure, Corrosion and Friction”. He has published more than 300 scientific articles and 20 book chapters, and his work has been cited more than 10.000 times. His research activities has been awarded by the AkzoNobel Science Award in 2013 and by the Arrhenius medal in 2008.
We are very pleased that Current Opinion in Colloid and Interface Science has decided to add a new section on surface forces. An understanding of surface forces is intertwined with the existing sections: it is well known that surface forces play a central role in determining the stability of colloidal dispersions, particle rheology, and the wetting of films, etc., but the topic has evolved into a discipline in its own right and thus it is fitting to have a section devoted specifically to surface forces. Advances in theoretical understanding and experimental techniques have together driven the development, so we include both aspects in in the section. Looking back at the study of surface forces, Amontons and Leonardo da Vince were already considering surface interactions in their empirical studies of friction during the middle ages, but
http://dx.doi.org/10.1016/j.cocis.2017.02.003 1359-0294/© 2017 Published by Elsevier Ltd.
real progress in understanding arrived much later. It is difficult to define where the modern era of surface force studies began, but the ground-breaking formulation of the Derjaguin, Landau, Verwey and Overbeek (DLVO) theory in the 1940’s, and the continuum theory of van der Waals forces presented by Lifshitz, Landau and coworkers in the 1950’s laid the foundation of modern interpretation. The theoretical understanding of surface forces generated by adsorbed polymer layers came with the advancement of scaling theories as formulated by deGennes and others, and by meanfield theories advanced by Scheutjens, Fleer, Vincent and Cosgrove in their classical “Polymers at Interfaces”, also known as the “blue book”. In addition, Monte Carlo and Molecular Dynamics simulations play an increasing role for enhancing the understanding of forces acting between surfaces. A major experimental breakthrough came with the development of the Surface Forces Apparatus and its description by Israelachvili and Adams in the 1970’s. Another important advancement was the invention of the Atomic Force Microscope by Binning, Quate and Gerber. These two techniques are still of major importance in the study of surface forces, but many others have also been developed to probe the intricate interactions between surfaces and particles in close proximity to each other under stationary conditions, or under normal or sliding motion. The papers presented in this Current Opinion issue reflect the many recent research directions, and they show that the area of surface force studies is as vibrant as ever, both in terms of exploring fundamental research issues and in terms of using studies of surface forces to gain insight into complex issues in industrial applications. Applications of surfaces forces are discussed in the contributions from Österberg and Valle-Delgado, which discusses surface forces in lignocellulosic systems, and in the contribution by Ivanova, Xu, Liu and Masliyah, which discusses surface forces in oil processing. In real life situations surfaces are most often rough and chemically heterogeneous, which is distinctly different compared to the ideal flat and homogeneous surfaces that most often are preferred in fundamental surface force studies. Thormann, in his contribution, discusses some of the issues arising from using rough surfaces in such measurements. There has been a notable trend in research away from equilibrium forces between hard, fixed objects to measurement of dynamic forces – friction, lubrication and frequency-dependent forces – and forces between soft or otherwise dynamic structures. This trend is reflected in the reviews.
A2
Editorial Overview – Surface Forces
For example, one scientific area that has seen enormous interest and rapid development in recent years is boundary lubrication in nanotribology contacts, where the close to frictionless motion of synovial joints has inspired and intrigued researchers across the world. This has led to coining of novel terms such as “hydration lubrication” and “lubrication synergy” to discuss and describe the findings. Briscoe addresses this area of research in his opinion contribution. Many of the biolubricants are polyelectrolytes, and some have complex architecture of e.g. the bottlebrush type. Polyelectrolytes, as mediators of surface forces, are in general intriguing and many-facetted since they interact both via electrostatic forces and other forces such as steric, bridging and depletion forces. Theoretical and modelling continues to advance, particularly the understanding of the effect of adsorbed polymers. In this first volume, we have two contributions in this area. Forsman discusses the current understanding of polyelectrolytes at surface and polyelectrolyte-induced forces, and Leermakers et al. discuss how dendron brushes alter frictional forces. The compact structure of dendrons reduces interpenetration, which in turn reduces friction. Reviews of responsive surfaces are also featured in this volume. The review from Butt et al. discusses mechanical contacts between an elastic solid and a stiff solid. Their review goes beyond JKR theory to consider dissipation, elastocapillary effects, hydrodynamic forces, and applications to hollow capsules. The review from the group of Frechette discusses similar effects, but the focus is on surfaces that are separated by fluid. In this case, the soft surfaces deform well before contact, producing dramatic changes in lubrication forces.
The original DLVO formulation for surface forces is still widely used and debated. We have included a review of recent work examining DLVO forces from Borkovic’s group. This review emphasises the influence of surface change regulation and multivalent ions. Pashley and Ninham review of the history of the study of surface forces, with emphasis on specific ion effects, as well as recent insights into bubble stability and evaporation from bubbles. Surface forces are not only of interest as such, but recent years have seen a development of AFM-based methodologies for elucidating material properties based on analysis of tip-surface interactions. On standard AFM instruments it is now possible to utilize modes that allow simultaneous collection of topography images and images that display material properties such as deformation, energy dissipation and Young’s modulus. Some of these properties can be determined without invoking any contact mechanics models, whereas others are extracted by fitting such models to collected force curves. Both approaches have advantages and disadvantages. There is also on-going efforts to develop and improve multi-frequency AFM techniques for material property mapping, extracted from tip-surface force measurements. Haviland addresses this rapidly developing research area in his contribution. Finally, we would like to thank all the contributors to this Current Opinion issue. It has been a great pleasure to read all the contributions, and it has stimulated many new thoughts. We hope and trust that you also will find inspiration from the articles and enjoy reading them.
Contents lists available at ScienceDirect
Current Opinion in Colloid & Interface Science journal homepage: www.elsevier.com/locate/cocis
Editorial Overview – Surface Forces William Ducker and Per Claesson
WILLIAM DUCKER William Ducker was born and educated in Australia, with a B.Sc. and Ph.D. from the Australian National University. He worked as a Post-Doctoral researcher at the University of California in Santa Barbara, a lecturer at the University of Otago and a Professor at the University of Melbourne. He is now at Virginia Tech in the USA. His research interests include: Surface Forces, Lubrication, Surface Organization, Atomic Force Microscopy, Stability of Colloids, Surfactants, Interface Thermal Conductance, and Bacterial Adhesion.
PER CLAESSON Per Claesson is professor in Surface Chemistry at the Royal Institute of Technology, KTH, Stockholm. He started his scientific career as PhD-student at KTH and during that time he enjoyed learning about surface forces during a one-year visit to The Australian National University, being exposed to the knowledge of Pashley, Israelachvili, Horn, Christenson and Ninham. Studies of surface forces have remained a central theme since that time. He has led several large collaborations involving academic and industrial partners, such as the competence centre “Surfactants Based on Natural Products” and the program on “Microstructure, Corrosion and Friction”. He has published more than 300 scientific articles and 20 book chapters, and his work has been cited more than 10.000 times. His research activities has been awarded by the AkzoNobel Science Award in 2013 and by the Arrhenius medal in 2008.
We are very pleased that Current Opinion in Colloid and Interface Science has decided to add a new section on surface forces. An understanding of surface forces is intertwined with the existing sections: it is well known that surface forces play a central role in determining the stability of colloidal dispersions, particle rheology, and the wetting of films, etc., but the topic has evolved into a discipline in its own right and thus it is fitting to have a section devoted specifically to surface forces. Advances in theoretical understanding and experimental techniques have together driven the development, so we include both aspects in in the section. Looking back at the study of surface forces, Amontons and Leonardo da Vince were already considering surface interactions in their empirical studies of friction during the middle ages, but
http://dx.doi.org/10.1016/j.cocis.2017.02.003 1359-0294/© 2017 Published by Elsevier Ltd.
real progress in understanding arrived much later. It is difficult to define where the modern era of surface force studies began, but the ground-breaking formulation of the Derjaguin, Landau, Verwey and Overbeek (DLVO) theory in the 1940’s, and the continuum theory of van der Waals forces presented by Lifshitz, Landau and coworkers in the 1950’s laid the foundation of modern interpretation. The theoretical understanding of surface forces generated by adsorbed polymer layers came with the advancement of scaling theories as formulated by deGennes and others, and by meanfield theories advanced by Scheutjens, Fleer, Vincent and Cosgrove in their classical “Polymers at Interfaces”, also known as the “blue book”. In addition, Monte Carlo and Molecular Dynamics simulations play an increasing role for enhancing the understanding of forces acting between surfaces. A major experimental breakthrough came with the development of the Surface Forces Apparatus and its description by Israelachvili and Adams in the 1970’s. Another important advancement was the invention of the Atomic Force Microscope by Binning, Quate and Gerber. These two techniques are still of major importance in the study of surface forces, but many others have also been developed to probe the intricate interactions between surfaces and particles in close proximity to each other under stationary conditions, or under normal or sliding motion. The papers presented in this Current Opinion issue reflect the many recent research directions, and they show that the area of surface force studies is as vibrant as ever, both in terms of exploring fundamental research issues and in terms of using studies of surface forces to gain insight into complex issues in industrial applications. Applications of surfaces forces are discussed in the contributions from Österberg and Valle-Delgado, which discusses surface forces in lignocellulosic systems, and in the contribution by Ivanova, Xu, Liu and Masliyah, which discusses surface forces in oil processing. In real life situations surfaces are most often rough and chemically heterogeneous, which is distinctly different compared to the ideal flat and homogeneous surfaces that most often are preferred in fundamental surface force studies. Thormann, in his contribution, discusses some of the issues arising from using rough surfaces in such measurements. There has been a notable trend in research away from equilibrium forces between hard, fixed objects to measurement of dynamic forces – friction, lubrication and frequency-dependent forces – and forces between soft or otherwise dynamic structures. This trend is reflected in the reviews.
A2
Editorial Overview – Surface Forces
For example, one scientific area that has seen enormous interest and rapid development in recent years is boundary lubrication in nanotribology contacts, where the close to frictionless motion of synovial joints has inspired and intrigued researchers across the world. This has led to coining of novel terms such as “hydration lubrication” and “lubrication synergy” to discuss and describe the findings. Briscoe addresses this area of research in his opinion contribution. Many of the biolubricants are polyelectrolytes, and some have complex architecture of e.g. the bottlebrush type. Polyelectrolytes, as mediators of surface forces, are in general intriguing and many-facetted since they interact both via electrostatic forces and other forces such as steric, bridging and depletion forces. Theoretical and modelling continues to advance, particularly the understanding of the effect of adsorbed polymers. In this first volume, we have two contributions in this area. Forsman discusses the current understanding of polyelectrolytes at surface and polyelectrolyte-induced forces, and Leermakers et al. discuss how dendron brushes alter frictional forces. The compact structure of dendrons reduces interpenetration, which in turn reduces friction. Reviews of responsive surfaces are also featured in this volume. The review from Butt et al. discusses mechanical contacts between an elastic solid and a stiff solid. Their review goes beyond JKR theory to consider dissipation, elastocapillary effects, hydrodynamic forces, and applications to hollow capsules. The review from the group of Frechette discusses similar effects, but the focus is on surfaces that are separated by fluid. In this case, the soft surfaces deform well before contact, producing dramatic changes in lubrication forces.
The original DLVO formulation for surface forces is still widely used and debated. We have included a review of recent work examining DLVO forces from Borkovic’s group. This review emphasises the influence of surface change regulation and multivalent ions. Pashley and Ninham review of the history of the study of surface forces, with emphasis on specific ion effects, as well as recent insights into bubble stability and evaporation from bubbles. Surface forces are not only of interest as such, but recent years have seen a development of AFM-based methodologies for elucidating material properties based on analysis of tip-surface interactions. On standard AFM instruments it is now possible to utilize modes that allow simultaneous collection of topography images and images that display material properties such as deformation, energy dissipation and Young’s modulus. Some of these properties can be determined without invoking any contact mechanics models, whereas others are extracted by fitting such models to collected force curves. Both approaches have advantages and disadvantages. There is also on-going efforts to develop and improve multi-frequency AFM techniques for material property mapping, extracted from tip-surface force measurements. Haviland addresses this rapidly developing research area in his contribution. Finally, we would like to thank all the contributors to this Current Opinion issue. It has been a great pleasure to read all the contributions, and it has stimulated many new thoughts. We hope and trust that you also will find inspiration from the articles and enjoy reading them.