Understanding and controlling the properties of fluid interfaces is relevant to develop materials of industrial interest (food stuffs, personal care products, cosmetics, pharmaceuticals, painting or lubrication), to design engineering processes, and to understand fundamental scientific problems.
There is a need to develop methodologies to understand, explain and predict the structural and dynamical properties of fluid interfaces. Computer simulations and theory (classical and ab initio computer simulations and classical density functional theory) are ideally positioned to complement experimental efforts based on diffraction and reflectivity techniques to answer these questions. As a matter of fact, recent theoretical and experimental studies have changed our traditional view of ion adsorption at aqueous interfaces. These and other studies underline the need to develop computational approaches that are more sensitive and provide an unambiguous description of the structure and dynamics of interfaces. The field has recently witnessed the development of new algorithms that tackle these questions. Although these algorithms are still under development they are providing a route to investigate the properties of nanoscopic interfaces at an unprecedented level of detail.
Interfacial functionalization is important to tune the properties of materials. The possibility of functionalizing interfaces with particles of different sizes is opening a new route to control the interfacial properties and to make a new class of soft materials. It has been acknowledged that the surfactant properties of colloids differ qualitatively from their molecular counterparts. The investigation of these functionalized interfaces is opening a number of challenges, firstly connected to interfacial degrees of freedom that are not present in the bulk, and secondly due the necessity of modeling the interface at different levels of detail, from atomistic to mesoscopic. Hence, different approaches are needed to cope with the wide range of length and time scales associated to these interfacial systems.
The aim of this Workshop Proposal is to bring together experts in computer simulation and theory of interfaces to establish the state of the art the theoretical investigation of the structure and dynamics of interfaces, as well as to delineate short-term objectives for the development of computational tools to assist in the design of functional interfaces. Recent computational developments in the simulation and theory of fluid interfaces make our proposal very timely. The Workshop should contribute to transfer these techniques to the experimental community, and with this purpose we have invited a number of experimental groups with expertise on structural and dynamic aspects of interfaces.
The computational investigation of interfaces has recently witnessed a number of important developments. The quantification of the interfacial structure via simulation is a long-standing problem that has evolved in parallel with experimental developments. Unlike solid surfaces, where it is well established that the surface properties determine adsorption and wetting phenomena, central to a number of industrial processes, we have lacked for a number of years, a similar level of understanding of liquid surfaces. This knowledge is nonetheless necessary as fluid interfaces separate phases that often feature very different compositions and chemical properties. Similarly, molecules moving from one phase to another will meet in the first instance the corresponding fluid surface. The need to attain a more detailed understanding of the structural and dynamic properties of the interfaces has motivated the development of new algorithms to compute the so-called intrinsic interfacial structure [1-4]. These algorithms are based on the definition of a so called 'intrinsic surface'. Such definition is not trivial. Unlike solid surfaces, fluid interfaces are mobile and undergo significant thermal fluctuations. Hence, several criteria have been proposed to define the interface location, which depending on the interface nature (liquid-vapour, liquid-liquid) can lead to different results. Hence, we want to motivate a discussion to establish the current limitations of different approaches and discuss future developments to obtain a unified approach for interfacial structure computations.
The approaches mentioned above have proved useful to investigate the limit of validity of hydrodynamic theory in the description of nanoscopic capillary waves . Similarly it is expected they will play in the near future an important role in solving controversial questions in fluid interfaces, such as the interfacial elastic properties of fluid interfaces [6,7] and the dependence of the interfacial free energy with the interface curvature [8-10]. The curvature is relevant in nanomaterials, particularly in nanoparticles, and it can significantly modify nanoparticle adsorption behavior . This question is receiving increasing attention, as particle adsorption represents a route to funtionalize interfaces, a route that is being exploited to stabilize emulsions and to synthesize nano and microstructures [12,13,14]. Interfacial functionalization with particles of different sizes has exposed the relevance of interfacial degrees of freedom, e.g., fluctuation forces, in determining the self-assembly behavior of the particles at the interface . At the same time recent work has exposed the limitations of traditional theoretical approaches of colloidal science in explaining the exotic behavior observed in charged colloids adsorbed at interfaces. There is evidence we need to extend current theoretical approaches to cope with the electrostatics of interfaces, which often separate media of very different dielectric permittivity. This result in a number of interesting effects, e.g., electro-capillary forces [11,13], and the peculiar partitioning of micro-ions at the oil-water interfaces . Simulations, density functional approaches in conjunction with experiments [17-19] have an important role to play in this area. To take full advantage of computer simulations in this area a better connection between atomistic / coarse grained and computational fluid dynamics approaches is needed. Recent studies have focused in the proper dynamic coupling of colloidal suspensions in binary mixtures, disregarding the relevance of molecular specificity on the adsorption properties of these soft materials.
Interfaces provide unique opportunities to enable new technologies. One of these is microfluidics, which is the base of 'lab-on-a-chip' technologies. The development of these technologies requires a substantial understanding of the hydrodynamic behavior of interfaces, as well as the mechanical properties of functionalized interfaces . In particular a microscopic understanding of the deformability of fluid-fluid interfaces can provide important clues on how red blood cells move in small capillaries, and also on important industrial applications such as the processing of emulsions and foams .
These Non-equilibrium effects at fluid interfaces, including heat and mass transfer processes at interfaces , have not been investigated in great detail using computational approaches. Hence, computational studies lag behind experimental and theoretical approaches.