Session X34: Emulsions, Foams, Gels, and Complex Fluids

Experimental measurement of the angle of repose of a pile of soft frictionless grains

It is well known that dry granular materials can flow like a liquid, but can also behave as a solid and sustain a finite angle of repose, partially as a result of inter-particle friction. Here we investigate the nature of piles formed with soft frictionless grains and measure its angle of repose. The pile is produced by a continuous bubbling of air into a soapy solution in a narrow container of rectangular cross section.~We observe a gentle slope at the water-foam interface whose angle dependents on the viscosity of the liquid. In contrast with sand piles, the fluidized region along the interface is several layers deep. We also find that, unlike sand piles, upon interruption of the gas flux, the slope relaxes back to zero as a result of bubble rearrangements and liquid drainage.   

Rearrangements during slow compression of a jammed 2D emulsion

We experimentally study non-affine motion within an evaporating quasi two-dimensional emulsion system. Our samples are oil-in-water emulsions confined between two close-spaced parallel plates, so that the oil droplets are deformed into pancake shapes. In this system, water slowly evaporates from an open edge of the chamber and, as a consequence, the volume fraction of oil droplets gradually increases. By means of microscopy, we analyzed the motion of droplets and measure the deformation of the droplet's outlines. Based on this information, we calculate the force network and the Voronoi cell when the system approaching jamming state. Using a recently proposed method (J. Rieser et al., arXiv:1509.05496), we calculate the Voronoi cell anisotropy vectors which point from the center of each particle to the corresponding Voronoi cell centroid, and identify void spaces where droplets may be more likely to move toward according to the field of the vectors. These allow us to study the correlations between the force network, the Voronoi vector field, and the non-affine displacements of droplets in our evaporating system.   

Self-Assembly of Emulsion Droplets into Polymer Chains

We experimentally investigate `beads-on-a-string' models of polymers using the spontaneous assembly of emulsion droplets into linear chains. Droplets functionalized with surface-mobile DNA allow for programmable 'monomers' through which we can influence the three-dimensional structure of the assembled 'polymer'. Such model polymers can be used to study conformational changes of polypeptides and the principles governing protein folding. In our system, we find that droplets bind via complementary DNA strands that are recruited into adhesion patches. Recruitment is driven by the DNA hybridization energy, and is limited by the energy cost of surface deformation and the entropy loss of the mobile linkers, yielding adhesion patches of a characteristic size with a given number of linkers. By tuning the initial surface coverage of linkers, we control valency between the droplets to create linear or branched polymer chains. We additionally control the flexibility of the model polymers by varying the salt concentration and study their dynamics between extended and collapsed states. This system opens the possibility of programming stable three-dimensional structures, such as those found within folded proteins.   

Cellulose Nanocrystals as Water in Water Emulsion Stabilizers

Cellulose is the most abundant polymer on the earth. Thus, it is very much desirable to find as many practical applications as possible for it. Cellulose, in its original form, contains both amorphous and crystalline parts. It is possible to separate both parts by dissolving the amorphous part in concentrated sulfuric acid. The remaining crystalline cellulose part exist in the form of rod-like particles. The dimensions of the particles depend on the source. We produce the particles from the acid hydrolysis of cotton cellulose fibers. It results in cellulose nanocrystals (CNCs) with dimensions of \textasciitilde 150 nm x 6 nm x 6 nm. It is well known that CNCs could very efficiently stabilize oil in water (O/W) emulsions by forming very dense monolayers of CNCs at O-W interfaces. However, it is not yet known whether they could also stabilize water in water (W/W) emulsions. The W/W emulsions can be produced by any two incompatible polymers. It is challenging to find effective stabilizers for W/W emulsions due to ultralow interfacial tension and large interfacial thickness. In this talk, I will show the efficiency and effectiveness of these one-dimensional rods as W/W emulsion stabilizers.   

Single droplet-level understanding of flow-induced phase inversion of emulsions

Phase inversion emulsification (PIE) is a process of generating emulsions by inverting the continuous and dispersed phases of a pre-existing emulsion. It is particularly useful when it is challenging to generate the target emulsions by conventional emulsification methods. Phase inversion of emulsions by flowing them through precisely engineered conduits is called flow-induced phase inversion emulsification (FIPIE). In this study a fundamental understanding of the underlying mechanism of FIPIE is developed. Phase inversion of monodisperse oil-in-water (O/W) emulsions into water-in-oil (W/O) emulsions is achieved by flowing them through specifically designed microfluidic channels. Based on in situ observation of single droplet-level events which lead to phase inversion, a mechanism of the process has been proposed. The outcome of the process is shown to depend on two dimensionless groups - Capillary number (relative importance of viscous and surface tension effects) and dimensionless droplet deformation (D/w, ratio of droplet size to channel width). It can be concluded from a state-plot between Ca and D/w that lower Ca and higher (D/w) facilitate FIPIE.   

Local Rearrangements of Droplets in a Dense Emulsion Under Shear Rearrangements

Jammed suspensions can flow when subjected to shear deformation. The flow properties are complex, depend on shear rates and can be inhomogeneous through the material. The microscopic origin of such flow properties is still a subject of intense research. In this work, we present a study of jammed emulsions under shear deformation, using a combination of experiments and molecular dynamics simulations. In the steady state regime, we investigate the local rearrangement of jammed emulsion droplets at a wide range of shear rates and shear strains and characterise the local rearrangement of droplets in terms of mean square displacement (MSD), displacement maps and displacement correlation function. At small shear strains and high shear rates, we find localised flow events and super diffusive motion of droplets. But at low shear rates, we observe emerging shear localisation from plastic events in an elastic background and avalanches. The characterisation of local rearrangements is also done in the stress over-shoot regime as well in the regime approaching the steady state stress. We observe a transient shear banding that progressively disappears as the pressure reaches a steady state value.   

Tuneable Rheological Properties of Fluorinated Pickering Emulsions

Pickering emulsions are an appealing approach to stabilize liquid-liquid dispersions without surfactants. Recently, amphiphilic silica nanoparticles have been proposed as an alternative to surfactants for droplet microfluidics applications, where aqueous drops are stabilized in fluorinated oils [1]. This system, proved to be effective in preventing the leakage of resorufin, a model dye that was known to leak in surfactant-stabilized drops[1][2]. The overall capabilities of droplet-based microfluidics technology is highly dependent on the dynamic properties of droplets, interfaces and emulsions[3]. Therefore, fluorinated pickering emulsions dynamic properties need to be characterized, understood and controlled to be used as a substitute of already broadly studied emulsions for droplet microfluidics applications. In this study, fluorinated pickering emulsions have been found to behave as a Herschel Bulkley fluid, representing a challenge for common microfluidic operations as re-injection and sorting of droplets. We found that this behavior is controlled by the interaction between the interfacial properties of the particle-laden interface and the bulk properties of the two phases. [1]M. Pan et al. ACS Appl. Mater 2014 [2]Y. Skhiri et al. Soft Matter 2012 [3]J.C. Baret, Lab Chip 2012   

A Computational Study of the Rheology and Structure of Surfactant Covered Droplets

Using different types of surface-active agents are ubiquitous in different industrial applications ranging from cosmetic and food industries to polymeric nano-composite and blends. This allows to produce stable multiphasic systems like foams and emulsions whose stability and shelf-life are directly determined by the efficiency and the type of the surfactant molecules. Moreover, presence and self-assembly of these species on an interface will display complex dynamics and structural evolution under different processing conditions. Analogous to bulk rheology of complex systems, surfactant covered interfaces will response to an external mechanical forces or deformation differently depends on the molecular configuration and topology of the system constituents. Although the effect of molecular configuration of the surface-active molecules on the planar interfaces has been studied both experimentally and computationally, it remains challenging from both experimental and computational aspects to track efficiency and effectiveness of different surfactant molecules with different molecular geometries on curved interfaces. Using Dissipative Particle Dynamics, we have studies effectiveness and efficiency of different surfactant molecules on a curved interface in equilibrium and far from equilibrium. Interfacial tension is calculated for linear and branched surfactant with different hydrophobic and hydrophilic tail and head groups with different branching densities. Deformation parameter and Taylor plots are obtained for individual surfactant molecules under shear flow.   

The origin of power-law rheology in foams

Soft glassy matter (SGM) such as foams, emulsions, and colloids, exhibit interesting rheological properties that have long defied explanation. In particular, the shear modulus of these materials displays weak power law frequency dependence. To understand the origin of this property in more depth, we have built a three-dimensional, modified Bubble Dynamics model. The bubbles interact with a purely repulsive harmonic potential and ripen according to diffusion-based governing equations. Notably, the bubble motion has a Levy flight character, in addition to being spatially correlated in the form of avalanches. Microrheology studies reveal that the power-law shear modulus is the result of constraint release driven by the bubbles' super-diffusive motion combined with simple yield of the resulting stress. The super-diffusive motion of the bubbles, in turn, is the result of the system taking a fractal path in configuration space. We shall discuss the origins of this fractal scaling.   

Creep dynamics in soft matter

Detecting any precursors of failure in Soft Matter Systems (SMS) is an inter-disciplinary topic with important applications (e.g. prediction of failure in engineering processes). Further, it provides an ideal benchmark to understand how mechanical stress and failure impacts the flow properties of amorphous condensed matter. Furthermore, some SMS are viscoelastic, flowing like viscous liquids or deforming like a solid according to applied forces. Often SMS are fragile and local rearrangements trigger catastrophic macroscopic failure. Despite the importance of the topic little is known on the local creep dynamics [1,2] before the occurrence of such catastrophic events [3,4]. To study creep and failure at an atomic/molecular level and at time scales that are not easily accessible by experiments we chose to carry out microscopic simulations. In this work we present the response of a colloidal system to uniaxial tensile stress applied and we compare our results to experimental works [8]. References: [1] Schurtenberger et al., J. Phys. Chem. 95, 4173 (1991). [2] Bauer et al., Phys. Rev. Lett. 97, 258303 (2006) [3] Chaudhuri P. et al., Phys. Rev. E 88, 040301 (2013). [4] Zausch J. et al., J. Phys. Condens. Matter 20, 404210 (2008).   

Modeling Discontinuous Phase Transitions in Gel Membranes: Focus on Hysteresis and Feedback Mechanisms

Feedback mechanisms are vital in a number of processes in biological systems. For example, feedback loops play an essential role during a limb development in mammals and are responsible for the asymmetric cell division to constrain the growth in plants to the specific regions. An integration of well-controlled feedback loops into the fully synthetic materials is an important step in designing a range of biomimetic functionalities. Herein, we focus on hydrogels functionalized with light-sensitive trisodium salt of copper chlorophyllin and study discontinuous phase transitions in these systems. Prior experimental studies had shown that illumination of these functionalized gels results in their heating and in discontinuous, first order phase transition upon the variation in temperature. Herein, we develop the first computational model for these gels; the framework of the model is based on the gel Lattice Spring Model, in this work we account for the gel heating under the illumination. The results of our simulations are in a good agreement with prior experimental studies. We focus on pattern development during the volume phase transitions in membranes of various thicknesses and show that one can effectively utilize light intensity to remotely control feedback loops in these systems.   

X-ray speckle measurements of concentrated nanoemulsions under shear

We present in situ X-ray Photon Correlation Spectroscopy (XPCS) measurements of a set of concentrated nanoemulsions subjected to oscillatory shear. The nanoemulsion set contained samples with varying packing fractions of oil droplets (r~20nm) above the jamming transition. In order to study their elasticity, yielding, and flow at various shear amplitudes, we employed stroboscopic coherent X-ray scattering measurements triggered at the maximums of the shear cycle. The degree of correlation between speckle in images taken a full period apart is a direct measurement of particle rearrangements during cycling. A comparison of such XPCS “echo” measurements with rheological measurements shows an onset of irreversible particle motion at shear strains below the crossover of the storage and loss moduli, which is typically used to indicate the transition to viscoplastic flow. Moreover, the XPCS echo measurements indicate that particle irreversibility increases rapidly with shear amplitude, in contrast to the comparably smooth transition to yielding shown in bulk rheology measurements. However, the macroscopic yield strain observed in rheology and the microscopic yield strain identified from XPCS, which were strong functions of droplet packing fraction, tracked each other closely.   

Dynamics of a DNA Gel

We study {\em in silico} the properties of a gel consisting of DNA strands (modeled as semi-flexible chains) and linkers of varying flexibility, length, and topology. These linkers are envisioned and modeled as active components with additional attributes so as to mimic properties of a synthetic DNA gel containing motor proteins. We use Brownian dynamics to directly obtain frequency dependent complex shear moduli of the gel. We further carry out force spectroscopy on these computer generated gels and study the relaxation properties as a function of the important parameters of the model, {\em e.g.}, densities and relative ratios of the DNAs and the linkers, the average life time of a link, etc. Our studies are relevant for designing synthetic bio-materials for both materials and medical applications.   

Supramolecular Structural Forces in Stratifying Foam Films and Micelle Aggregation Number

Understanding and controlling the drainage kinetics of thin films is an important problem that underlies the stability, lifetime and rheology of foams and emulsions. Foam films containing micelles, colloidal particles or polyelectrolyte-surfactant mixtures exhibit step-wise thinning or stratification, due to the influence of non-DLVO forces, including supramolecular oscillatory structural forces. In this study, we use experiments and theory to investigate the drainage and stratification in vertical and horizontal thin foam films (\textless 100 nm) formed by aqueous sodium dodecyl sulfate (SDS) solutions.~We determine how the concentration of surfactants and added salt influences the stepwise thinning process for micellar solutions, and how step size can be used for estimating micelle size and interactions. The concentration-dependent aggregation number extracted from our experiments match-up reasonably well with values obtained by other techniques including scattering and fluorescence.   

Microrheology using a custom-made AFM

In the past few years, a new method was developed to measure local properties of liquids (X. Xiong \textit{et al.}, Phys. Rev. E 80, 2009). This method consists of gluing a micron-sized glass fiber at the tip of an AFM cantilever and probing the liquid with it. In ENS Lyon, this method was perfected (C. Devailly \textit{et al.}, EPL, 106 5, 2014) with the help of an interferometer developped in the same laboratory (L. Bellon \textit{et al.}, Opt. Commun. 207 49, 2002 and P. Paolino \textit{et al.}, Rev. Sci. Instrum. 84, 2013), which background noise can reach $ 10^{-14}\ m/\sqrt{Hz} $. This method allows us to measure a wide range of viscosities ($ 1\ mPa.s$ to $ 500\ mPa.s $) of transparent and opaque fluids using a small sample volume ($ \sim 5\ mL $). In this presentation, I will briefly describe the interferometer developped in ENS Lyon, then explain precisely the microrheology measurements and then compare the experimental results to a model developped by M. Benzaquen.