Editorial: dynamics and rheology of complex fluid–fluid interfaces

Jan Vermant, K.U. Leuven, Belgium (left) and Gerald G. Fuller, Stanford University, USA (right)

Complex, fluid–fluid interfaces surround us. We encounter them in the personal care products we enjoy, they structure the foods we eat, and they are the building blocks of living organisms. Since they are mobile and deformable, their rheology and microstructure in response to interfacial flows become an essential part of understanding a variety of industrial and natural processes. Naturally occurring events controlled by interfacial rheology include the process of cell division, the folding of a red blood cell into the shape of a Crêpe Suzette to negotiate small capillaries,¹ or the stability of the tear film that protects our eyes.² Industrial applications arise from the requirement of stabilizing emulsions and foams that comprise personal products and foods. By rendering an oil/water interface viscoelastic, the stability of emulsions and foams can be enhanced,³ and this can be accomplished through the strategic addition of molecular amphiphiles, proteins or particles. The environment is impacted when biofilms form or when an oil spill results in emulsions that resist phase separation. The strength of such emulsions is explained by the presence of asphaltenes in heavy oils that lodge at oil/water interfaces and generate viscoelastic skins on the surface of droplets.⁴

Fluid–fluid interfaces display such rich, nonlinear flow phenomena because of the tendency of surface active species to self-organize and often combine to create viscoelastic surfaces that possess such non-Newtonian attributes as elastic moduli, shear thinning surface viscosities, long relaxation times, and yield stresses. Like their corresponding bulk, complex fluid materials, these fluid interfaces give rise to similar classifications. Two-dimensional polymer melts composed of flexible, amphiphilic chains laying flat on a fluid interface display many of the non-Newtonian fluid mechanical responses that are familiar to their three-dimensional counterparts⁵ and yet cannot entangle in the same manner.⁶ The interactions existing between particles at interfaces can be manipulated to create a rich variety of microstructures ranging from aggregated, linear chains to deformable two-dimensional crystals.⁷ Exquisite control of particle morphology and surface chemistry makes it possible to generate extended Rouleaux in the case of ellipsoidal particles.^8,9 These particle-laden interfaces find important application in the stabilization of emulsions and foams.¹⁰ Introducing paramagnetism to the particles leads to emulsions that can be manipulated and destabilized by external magnetic fields.¹¹ A similar strategy leading to temperature-controlled emulsions can be followed by using thermo-responsive, poly(N-isopropylacrylamide) gel particles as stabilizers.¹² Proteins at interfaces form another class of complex, fluid–fluid interfaces and are the basis for the beneficial stabilization of many food emulsions.¹³ These macromolecules, with alternating hydrophobic and hydrophilic patches, are able to reconfigure themselves through denaturation and, when this occurs, surface gelation can ensue. However, in the manufacture of monoclonal antibodies, this occurrence leads to the detrimental appearance of unwanted particulates. Finally, the interfacial dynamics of classical amphiphiles such as fatty acids, fatty alcohols, and phospholipids reveal rich, nonlinear responses. This family of amphiphiles has the ability to self-assemble in long range, ordered lattices. Consequently, many of the phases of these molecules respond to flow in ways that are reminiscent of smectic liquid crystals.¹⁴

The emerging field of interfacial rheology requires the development of experimental methods that can properly isolate interfacial rheological material functions. These methods can be broadly divided into devices that operate at approximately constant surface area and those that provide dilatational properties. The former class primarily offers simple-shear surface rheology and include gliding magnetic rods,¹⁵ double wall rings¹⁶ and biconical disks,¹⁷ as recently reviewed by Miller et al.¹⁸ The success of these instruments has, in many cases, led to their commercialization. More recently, active micro-rheological methods have been developed to probe local structure.¹⁹ Coincident with the development of these mechanical measurement tools, optical methods for the extraction of interfacial microstructure have been successfully applied to these systems (epifluorescence, Brewster angle, and optical microscopies, and dichroism). These optical techniques reveal a rich variety of flow-induced effects, such as jamming in two-dimensional suspensions,²⁰ orientation of two-dimensional liquid crystals, and annealing of polydomain structures in fatty acids.²¹ Dilatational surface rheometry has primarily relied on oscillating pendant drops and bubbles²² and provides information of direct importance to the mechanics of the alveoli of the lung and Ostwald ripening.

The present themed issue on “Dynamics and Rheology of Complex Fluid–Fluid Interfaces” showcases the variety of research challenges that are currently being addressed. Several contributions discuss new experimental methods to investigate these systems and their dynamics. The techniques include single molecule microscopy, optical tweezers, as well as micro- and macroscopic methods to determine the rheological properties in shear, extension and dilatation (DOI: 10.1039/c1sm05263b, DOI: 10.1039/c1sm05256j, DOI: 10.1039/c1sm05232b, DOI: 10.1039/c1sm05235g, DOI: 10.1039/c1sm05262d, DOI: 10.1039/c1sm05254c, DOI: 10.1039/c1sm05253e, DOI: 10.1039/c1sm05383c, DOI: 10.1039/c1sm05255a, DOI: 10.1039/c1sm05399j). A fundamental challenge in dealing with rheology and fluid mechanics in the presence of complex fluid–fluid interfaces, is the intimate coupling between bulk and surface flows. The deformation of an interface, such as that which occurs in capillary leveling flow, can be exploited to measure the ‘bulk’ properties of confined liquids (DOI: 10.1039/c1sm05261f, DOI: 10.1039/c1sm05340j, DOI: 10.1039/c1sm05634d, DOI: 10.1039/c1sm05258f). On the other hand, the presence of a complex fluid–fluid interface can lead to interesting interfacial fluid dynamics (DOI: 10.1039/c1sm05271c, DOI: 10.1039/c1sm05144j, DOI: 10.1039/c1sm05462g, DOI: 10.1039/c1sm05205e). For example, the interplay of interfacial viscoelasticity with drying and wetting phenomena contributes to a rich variety of physical phenomena (DOI: 10.1039/c1sm05231d, DOI: 10.1039/c1sm05220a).

As in the case of bulk soft matter, there are several material classes that are currently being studied. Particle laden interfaces have been studied extensively in recent literature and this themed issue is no exception (DOI: 10.1039/c1sm05248a, DOI: 10.1039/c1sm00005e, DOI: 10.1039/c1sm05257h, DOI: 10.1039/c1sm05240c, DOI: 10.1039/c1sm05457k, DOI: 10.1039/c1sm05149k, DOI: 10.1039/c1sm05407d). The diversity of complex fluid interfaces studied is significant and includes proteins and protein aggregates, various types of oligomers, polymers, and even soft matter composite interfaces (DOI: 10.1039/c1sm05357d, DOI: 10.1039/c1sm05338h, DOI: 10.1039/c1sm05225j, DOI: 10.1039/c1sm05234a, DOI: 10.1039/c1sm05421j, DOI: 10.1039/c1sm05349c, DOI: 10.1039/c1sm05378g). The behaviour of these interfaces mimics the phenomena normally associated with bulk materials, such as non-Newtonian effects, soft glassy responses and even highly non-linear rheology, but can be further complicated by adsorption–desorption dynamics.

As the different contributions show, complex fluid–fluid interfaces represent an emerging field of study in soft matter physics. Yet, exciting opportunities and challenges lie ahead. The non-Newtonian interfacial velocity patterns associated with two-dimensional polymer melts suggest that these systems produce normal stress differences, but no direct measurement of this manifestation of surface viscoelasticity has been developed. Similarly, extensional surface flow gradients at constant surface area are expected to lead to interfacial extensional viscosities that could be as dramatic as the corresponding effects seen in the bulk. However, much work is required to ferret out the presence of two-dimensional extensional flow properties. Although dilatational measurements using oscillating bubbles and drops provide substantial and useful information, the use of the Young–Laplace equation in the analysis of this method can be problematic, especially for highly elastic interfaces. Techniques that do not rely on the use of that equation would be an advantage.

As experimental methods improve our understanding of the relationship between interfacial microstructure and rheology, there is a pressing need for the development of microscopic constitutive equations. Past attempts have often been directly borrowed from successful models developed for bulk materials, such as in the case of two-dimensional liquid crystals. However, there are many features of two-dimensional systems that do not simply carry over, such as the nature of chain–chain interactions in two-dimensional polymer melts, the description of particle–particle interactions at fluid interfaces, and the dynamics of self-assembled monolayers.

We invite readers to become acquainted with the remarkable behavior of complex fluid–fluid interfaces as revealed by the papers assembled in this special issue. There are compelling phenomena to investigate in this branch of soft matter physics.

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