Session M32: Physics of Complex Liquid Interfaces

Interfacial Rheology and Breathing

The dependence of the Laplace pressure, ΔP = 2γ/R, on alveolar radius, R, means that interconnected alveoli are metastable if γ is constant. However, while not generally appreciated in the medical literature, but well known in the foam and emulsion stability literature, the dynamic resistance of an interfacial film to compression can reverse the Laplace instability. The dilatational modulus, ε=A∂γ/∂A , relates the change in surface tension, γ, to the change in molecular area, A, as the interface is compressed at frequency, ω (ranging from 1-20 radians/second for normal breathing). If the dilatational modulus is large enough, the resistance to interfacial compression can overcome the Laplace pressure so that the gas pressure in the alveolus no longer increases with decreasing radius. For (2ε-γ) > 0, the Laplace pressure decreases with decreasing radius and increases with increasing radius, which reverses the Laplace instability, thereby stabilizing the alveoli against collapse. Under normal conditions, lung surfactant generates conditions such that (2ε-γ) > 0, and the lung remains stable. However, during Acute Respiratory Distress Syndrome, trauma or disease leads to a dramatic increase in the concentration of albumin and lysophosphatidylcholine, soluble surface-active molecules that compete for the interface with lung surfactant. Using a newly designed capillary microtensiometer, we have found that increasing concentrations of lysophosphatidylcholines, a product of the inflammation induced degradation of phospholipids, causes the dilatational modulus to decrease as ω decreases, resulting in (2ε-γ) < 0, creating conditions that induce the Laplace instability. This suggests a mechanism underlying ARDS which kills 50,000 people each year with no known cure. Increasing the breathing frequency or decreasing the lysophosphatidylcholine concentration can increase the dilatational modulus and may restore proper lung function in ARDS.   

Solutal Marangoni spreading in the presence of pre-deposited insoluble surfactant monolayers

Marangoni spreading can be used for pulmonary drug delivery. However, endogenous surfactant in the lung may inhibit that spreading. Here we investigate how the density of pre-existing phospholipid monolayers impacts Marangoni flows induced by exogenous surfactants on a thin fluid subphase. A dipalmitoylphosphatidylcholine (DPPC) monolayer is pre-deposited at a predetermined lateral density on an aqueous subphase before depositing a drop of oleic acid, an insoluble surfactant. Talc tracer particles monitor the extent of spreading, and a trans-illumination method is used to measure the temporal evolution of the subphase surface deformation. This reveals the outwardly moving “Marangoni ridge”. As long as the surface pressure of the DPPC monolayer is less than that of an oleic acid monolayer, spreading proceeds until the surface pressures of the monolayers are equal. The final area per molecule of DPPC in the compressed monolayer is thus the same in each case. The trajectory and degree of interface deformation caused by Marangoni flow are altered by increasing initial DPPC concentration. Unlike Marangoni flow on bare subphases, there is surface flow ahead of the Marangoni ridge as the DPPC monolayer is compressed.
  

Exploring the free energy landscape to predict surfactant adsorption isotherm at nanoparticle-water interface

The long-lasting stability of nanoparticle (NP) suspensions in aqueous solutions is commonly reached by the addition of surfactants. However, a tailored prediction of surfactant concentration enabling a well dispersion of NPs is still an ambitious objective (Cardellini et al., 2019). Here, by coupling Steered Molecular Dynamics (SMD) with the Langmuir theory, we predict the adsorption isotherm of Sodium-Dodecyl-Sulphate (SDS) on α-alumina NP. Beyond the design, our results unveil the role of NP curvature and morphology in the adsorption phenomena at solid-liquid interface. First, the Free Energy Landscapes (FELs) obtained via SMD show the dominance of the entropic contribution by increasing the SDS molecules on target bare NP. Second, a thorough analysis on the FELs, clarifies how the enhanced adsorption deviates from a Markovian dynamics in a curved interface. In fact, both NP curvature and morphology promote modification of the thermodynamics state of adsorption with consequent free energy profiles splitting and the identification of specific sites of adsorption. The proposed modelling framework provides physical insights in the surfactant adsorption onto spherical NPs and suggests guidelines to design NP suspensions in aqueous solutions.
(De Angelis et. al, accepted on ACScs).   

Finite-size line tension effects for nanoparticles at the air-liquid surface

The line tension of nanoparticles, at the air-liquid surface, can be deduced by examining the variation in surface tension with nanoparticle bulk concentration. The line tension of dodecane thiol ligated Au nanoparticles at the octadecane-air surface (at 30C) is found to vary with nanoparticle size. This finite-size line tension effect is qualitatively explained by integrating over the van der Waals contributions to the line tension at the three-phase solid-liquid-air contact line [1].

[1] H. Matsubara, J. Otsuka, and B. M. Law, Langmuir 34, 331 (2018).   

Adsorption of nanosheet particles to surfactant-laden interfaces

It is known that particles with appropriate wettability spontaneously adsorb to neat water-oil interfaces. The combined effect of particles and surfactants at interfaces, however, is still poorly understood. In this work, we show that selected oil-soluble surfactants promote the adsorption of hydrophilic nanosheet particles and enhance bulk oil-in-water emulsion stability. Using microfluidics, we evaluate interface stability against coalescence and the competitive adsorption dynamics of particles versus surfactant for nanosheets of varying degrees of surface activity.   

What are the effects of processing conditions on the interfacial viscoelasticity of asphaltene interfaces?

Stable crude/water emulsions that form during processing result in the formation of the rag layer, which is a major detriment to petroleum processing. The stability of the rag layer is determined by drop resistance to coalescence. Interfacial asphaltenes and other surface active components create viscoelastic interfaces that inhibit coalescence, resulting in stable rag layers. Although, there have been efforts into studying asphaltene interfaces, there are still many open questions regarding their behavior. In particular, the formation pathway of the asphaltene interface on final interfacial elasticity.

We have examined how large amplitude dilatational strains change the interfacial shear viscoelasticity of asphaltene interfaces. We see changes in viscoelastic moduli that depends on how the interface was formed: adsorption, larger single compression, or multistep small compressions. Interfaces that adsorb show larger viscoelastic moduli than single step compression interfaces, but smaller values than multistep compression. Small interfacial expansions result in minimal changes of the interface, but large expansions significantly disrupt surface structure creating weak interfaces. These results shed important light on how processing of these interfaces may affect rag layers.   

Pore scale, multi phase flow assessment for industrial applications

We report progress of our research of flow analysis at nanometer to micrometer scales for industrial applications. For an investigation of oil recovery at pore scale, digital representations of capillary networks in reservoir rocks are derived from data obtained by applying microscopic computer tomography to representative rock samples. The capillary network representations derived from digital rock are used as geometric boundaries for numerical flow simulations of complex fluids containing water, oil, and additives such as polymers or surfactants. The hierarchical, multiphase flow models deployed in the numerical simulations are calibrated at nanometer scale through molecular dynamics simulations. Importantly, the flow simulations are experimentally verifiable by means of a nanofluidic rock-on-chip measurement platform integrated with semiconductor technology. The laboratory method allows extracting physical parameters that determine surface wetting and capillary flow properties at microscopic scales. The methods developed in our research are more generally applicable to problems of fluid flow in porous media. Their combined application will increase accuracy and enable cost and time reduction in permeability assessment for exploration and production of natural resources.   

2D to 3D transition of nanoparticles assembled at the liquid interface

With in situ atomic force microscopy (AFM) imaging, the structure of nanoparticles assembled at the water-oil interface is clearly resolved with the nanometer in 3D. The increase in the surfactant concentration in the oil phase leads to the increase in the contact angle of the nanoparticles at the water-oil interface. Unlike the prediction by equilibrium theory, a 2D monolayer to a 3D multilayer transition of nanoparticles is observed at high surfactant concentrations, which is attributed to the co-existence of completed wetted and partially wetted nanoparticles at the water-oil interface. The contact angle change and structural transition of nanoparticles induced by the increase of surfactant concentration is further confirmed macroscopically by the phase-inversion of Pickering emulsions and anomalous compliance of liquid interface. Our study sheds light on the importance of both thermodynamics and kinetics on the assembly of nanomaterials at the liquid interface and also demonstrate the advantage of using in situ AFM to characterize the assembly of nanomaterials at the liquid interface.   

Co-surfactant based interfacial strategies for the formation and stabilization of multiple nanoemulsions

There has been significant recent interest in the production of multiple emulsions – i.e., droplets in droplets – of increasingly small size, especially at the nanoscale. However, the energetic cost of stabilizing many highly curved interfaces makes it challenging to produce and stabilize such multiple nanoemulsions. Here, we propose a general method for promoting and stabilizing complex nanodroplet structures by manipulating their interfacial mechanics through co-surfactants of opposing spontaneous curvature. Using asymmetric pairs of ethoxylated co-surfactants, we experimentally show that this strategy induces the preferential formation of droplets with multiple highly curved interfaces and develop a theoretical framework to predict the observed droplet structures. Using neutron spin echo experiments, we measure the equilibrium dynamics of these ultra-low surface tension systems to determine the effective elastic constants of the mixed surfactant membranes. Using the measured elastic constants, we develop equilibrium interfacial free energy models to quantify conditions under which multiple nanoemulsion structures are stabilized, providing a rational means for engineering co-surfactant systems to promote the formation and stability of multi-phase nanodroplets.   

High-speed X-ray Photon Correlation Spectroscopy studies of dynamics in liquid-liquid extraction systems

X-ray Photon Correlation Spectroscopy (XPCS) and newly available high-speed X-ray detector systems provide a golden opportunity for shedding new light on a problem of both practical and fundamental importance – the dynamics of fluctuations near critical points in complex fluids. Here, we present a high-speed XPCS study of critical fluctuation dynamics in an important class of complex organic fluids designed for chemical separations. In these liquid-liquid extraction systems, an amphiphilic extractant molecule is used to selectively transfer targeted species from an aqueous phase into an organic phase through the formation of nanoscale molecular complexes. The most efficient extraction typically occurs near a critical point where the correlation length of the complexes becomes large. Our XPCS studies reveal the microsecond dynamics of the molecular assemblies both away from and at the critical temperature T_c. The observed fluctuation dynamics can be characterized as a function of Q and T using predictions from the theory of critical phenomena. In particular, we see that the dynamic critical exponent z describing the Q dependence of tau ~ Q^-z changes from 2 to 3 as the critical temperature Tc is approached   

Tubulation and dispersion of oil by growth of marine bacteria on oil droplets

The activity of bacteria on surfaces exhibits collective behaviors such as active turbulence and active stresses, which result from motility or growth and interactions with the local surroundings. Here, we describe experimental observations on the emergence of tubular structures resulting from the growth and division of rod-shaped bacteria on the liquid interface of oil droplets. Using microfluidics and time-lapse microscopy, we quantify the dimensions and growth rates of individual tubular structures as well as the bulk biofilm formation on hundreds of droplets over 72 hours. We find that the length of tubular structures is comparable to the initial droplet radius and that they are composed of an outer shell of bacteria that stabilize an inner filament of oil. The oil filament undergoes breakup into smaller microdroplets dispersed within the bacterial shell, where the average radius of the dispersion is described by the most unstable Plateau-Rayleigh wavelength. This work provides insight into active stresses at deformable interfaces and improves our understanding of microbial oil biodegradation and its potential influence on the transport of oil droplets in the ocean water column.   

Effect of polymer topology on the structure and dynamics of block copolymers at Liquid/Liquid Interfaces

The behavior of molecules such as polymers at liquid/liquid interfaces is both scientifically and technologically important and has been the focus of much attention in recent years given the various types of materials that have been successfully assembled at liquid/liquid interfaces. Polymers do assemble in various ordered aggregates at liquid/liquid interfaces and understanding the effect of polymer topology on the overall behavior of a given polymer at surfaces and interfaces and especially at liquid/liquid interfaces is very important. Molecular dynamics simulations using a bead-spring model on representative block copolymer architectures at immiscible liquid/liquid interfaces have been carried out. The results of our detailed investigation on the structure and dynamics of the block copolymers at the interface will be presented and discussed.
  

A novel method for interfacial rheology using an indirect interfacial rheometer

We have developed a system for performing interfacial rheology without attaching a probe directly to the interface itself. This indirect rheology is motivated by the applications of Pickering type systems, where rather than shear being applied directly to an interface, one of the continuous phases is sheared. The behaviour of the system as a whole is then governed, in part, by the rheology of that interface.
We use this setup to measure flow curves in a system of poly(methyl methacrylate) particles at a water/dodecane interface. The lack of a direct probe ensures two things: 1) we avoid substantial pre-shearing and 2) the response measured is due to the interface itself and not the probe [1].
In this indirect setup a shear is applied to the upper oil phase using a standard stress controlled rheometer. The response of the interface is then measured by confocal microscopy. A range of rheological behaviour is observed dependent on surface coverage, shear rate and surface structure. The direct visualisation of the interface allows all of these input parameters to be measured and various flow curves can be produced to observe their effect on the rheology of the interface.
[1] R. Van Hooghten et al, Langmuir 33, 4107 (2017)