Emulsion

Emulsion stability

Review

Emulsions have been studied for a long time because of the richness of their related fundamental physicochemical phenomena and owing to their wide industrial applications. The development of microfluidics offers new opportunities to investigate emulsion features and behaviours. This review relates the use of microfluidic tools for probing the interfacial properties of emulsion droplets and the two main mechanisms of destabilisation, namely the coalescence of adjacent drops and the molecular transfer between neighbouring drops.

N. Bremond, J. Bibette, 2012, Exploring emulsion science with microfluidics. Soft Matter, 8, 10549-10559.

Separation-driven coalescence: experiment

Symmetrical fusion

The destabilization process of an emulsion under flow is investigated in a microfluidic device. The experimental approach enables us to generate a periodic train of droplet pairs, and thus to isolate and analyze the basic step of the destabilization, namely the coalescence of two droplets which collide. We demonstrate a counter intuitive phenomenon: coalescence occurs during the separation phase and not during the impact. Separation induces the formation of two facing nipples in the contact area that hastens the connexion of the interfaces prior to fusion. Moreover, droplet pairs initially stabilized by surfactants can be destabilized by forcing the separation.
Finally, we note that the fusion mechanism is responsible of a cascade of coalescence events in a compact system of droplets where the separation is driven by surface tension.

N. Bremond, A. R. Thiam, J. Bibette, 2008, Decompressing emulsion droplets favors coalescence. Phys. Rev. Lett., 100, 024501.

Physical Review Focus

see also the story in Physical Review Focus

Separation-driven coalescence: theory

Recent microfluidic experiments by Bremond et al. (2008), along with simulations by Yoon et al. (2007) as well as near contact experiments and simulations by Manica et al. (2008), have demonstrated that two droplets can coalesce as they are separating rather than upon their collision. We analyze the experimental microfluidic flow configuration for the approach to contact with a two-dimensional model: we apply a lubrication analysis followed by the method of domain perturbation to determine the droplet deformation as a function of time. We find the approximate shape for the deformed droplet at the time of contact. In particular, for droplets of radius, R, moving apart according to h_0(t)=h_0(0)+\alpha t^2, where 2h_0(t) is the separation distance, we define a non-dimensional parameter, A=\frac{4C\mu R^2\alpha^{1/2}}{\pi \gamma [h_0(0)]^{3/2}}, where \mu is the viscosity of the continuous phase, \gamma is the interfacial tension, and C depends on the viscosity ratio between the droplets and the continuous phase. Our model suggests that there exists a critical value, A_{crit}=\frac{16}{3^{3/2}} ~ 3.0792, below which separation is unlikely to facilitate the coalescence of the droplets. The predictions are in good agreement with available experimental data.

A. Lai, N. Bremond, H. A. Stone, 2009, Separation-driven coalescence of droplets: An analytical criterion for the approach to contact. J. Fluid Mech., 632, p. 97-107.

Coalescent coalescence: a novel scenario of phase inversion

propagation2D

The phase inversion that undergoes an emulsion while being sheared is a sudden phenomenon that is still puzzling. In this Letter, we report an experimental investigation on propagative coalescence by using a microfluidic device where a calibrated two-dimensional emulsion is created and destabilized. The velocity of propagation as well as the probability of the coalescence are reported as a function of the size and the spatial distribution of the drops. We then discuss the efficiency of this novel scenario of phase inversion and suggest that inversion can be favored by the existence of a drop size distribution.

N. Bremond, H. Doméjean, J. Bibette, 2011, Propagation of drop coalescence in a two-dimensional emulsion: A route towards phase inversion. Phys. Rev. Lett., 106, 214502.

Electrocoalescence

Electrocoalescence

By using microfluidic chips, we investigate the stability regarding coalescence of droplet pairs under electric field as a function of drop separation and ac field intensity. Three different regimes are found: stable, coalescence and partial merging. From this, we identify the two breaking scenarii of a one dimensional train of droplets: in one case the coalescence front propagates, in the other case, which for pairs corresponds to the partial merging regime, the coalescence front can become heterogeneous. From these findings, we can propose a destruction mechanism for a macroscopic emulsion, which includes the packing condition for which total and immediate destruction is effective.

A. R. Thiam, N. Bremond, J. Bibette, 2009, Breaking of an emulsion under an ac electric field. Phys. Rev. Lett., 102, 188304.

Adhesive emulsions

AdhesivePair

Water-in-oil emulsion drops are formed and stabilized with phospholipids which can adhere and form a bilayer. By using microfluidic technologies, we probe the stability and properties of such membranes likewise encountered in foams or vesicles. First, adhesive drop pairs are then trapped and submitted to an ac electric field. We observe three distinct states as a function of the adhesion energy and the electric field intensity. The pair can be either stable, though slightly deformed, or unzip and separate, or coalesce. The frontiers between the different states directly reflect vesicle detachment forces and electroporation theories. The experimental approach that we propose for probing liquid interface wetting between monolayers allows us to finely tuned the tension in the bilayer and gives access to bilayer unzipping. We then establish the stability diagram of adhering drop pairs as a function of the continuous phase composition. We found two regimes of destabilization of the bilayer. The first one concerns a competition between the dynamics of adhesion and the transport of surfactants towards the interfaces that leads to a dilute surfactant coverage. The second one corresponds to a dense surface coverage where the lifetime distribution of the bilayer exponentially decreases as a signature of a nucleation process. In the stable regime, we observe the propagation of adhesion among a concentrated collection of drops. This is another remarkable illustration of the suction consequence when two close deformable objects are pulled apart. Moreover, the present experimental strategy offers a novel way to study the phase diagrams of bilayers from a single phospholipid to a mixture of phospholipids. Indeed, we detect phase transitions at a liquid-liquid interface that are ruled by the amount of bad solvent. Finally, we probe the transport of water molecules through the bilayer and show that its permeability is linked to the adhesion energy that reflects its fluidity.

A. R. Thiam, N. Bremond, J. Bibette, 2011, Adhesive emulsion bilayers under an electric field : from unzipping to fusion. Phys. Rev. Lett., 107, 068301.

A. R. Thiam, N. Bremond, J. Bibette, 2012, From stability to permeability of adhesive emulsion bilayers. Langmuir, to be published.

Microorganisms and digital micro/milli-fluidics

This work, which is the framework of the PhD project of Laurent Boitard and Fabien Bertholle at LCMD, relies on the use of emulsion droplets as reservoirs in which micro-organisms evolve. The encapsulation of micro-organisms in thousands of identical droplets that are probed in the course of time, should enable us to quantify the noise related to their development (e.g. the distribution of the cell division time). Microfluidic approach is used for probing this noise at the level of a single cell and the millifuidic technology for the behavior at the level of a small population. Those projects are conducted in close collaboration with biologists: Gaël Yvert from ENS Lyon and Arjan de Visser from the University of Wageningen, The Netherlands.

Cell Bioenergetics and Coarsening of Emulsion Droplets

shrinkYeastDroplet

Microorganisms are widely used to generate valuable products and their efficiency is a major industrial focus. Bioreactors are typically composed of billions of cells and available measurements only reflect the overall performance of the population. However, cells do not equally contribute and process optimization would therefore benefit from monitoring this intra-population diversity. This has so far remained difficult because of the inability to probe concentration changes at the single cell level. Here, we unlock this limitation by taking advantage of the osmotically driven water flux between a droplet containing a living cell towards surrounding empty droplets, within a concentrated inverse emulsion. With proper formulation, excreted products are far more soluble within the continuous hydrophobic phase compared to initial nutrients (carbohydrates and salts). Fast diffusion of products induces an osmotic missmatch, which further relaxes due to slower diffusion of water through hydrophobic interfaces. By measuring droplet volume variations, we can deduce the metabolic activity down to isolated single cells. As a proof of concept, we present the first direct measurement of the maintenance energy of individual yeast cells. This method does not require any added probes and can in principle apply to any osmotically sensitive bioactivity, opening new routes for screening and sorting large libraries of microorganisms and biomolecules.

L. Boitard , D. Cottinet, C. Kleinschmitt, N. Bremond, J. Baudry, G. Yvert and J. Bibette, 2012, Monitoring Single Cell Bioenergetics via the Coarsening of Emulsion Droplets. Proc. Natl. Acad. Sci. USA, to be published.

Millifluidic droplet analyser

dropTrainMilli

We present a novel millifluidic droplet analyser (MDA) for precisely monitoring the dynamics of microbial populations over multiple generations in numerous (≥10^3) aqueous emulsion droplets (~100 nL). As a first application, we measure the growth rate of a bacterial strain and determine the minimal inhibitory concentration (MIC) for the antibiotic cefotaxime by incubating bacteria in a fine gradient of antibiotic concentrations. The detection of cell activity is based on the automated detection of an epifluorescent signal that allows the monitoring of microbial populations up to a size of ~106 cells. We believe that this device is helpful for the study of population dynamic consequences of microbe-environment interactions and of individual cell differences. Moreover, the fluidic machine may improve clinical tests, as it simplifies, automates and miniaturizes the screening of numerous microbial populations that grow and evolve in compartments with a finely tuned composition.

L. Baraban, F. Bertholle, M. L.M. Salverda, N. Bremond, P. Panizza, J. Baudry, J. A. G.M. de Visser and J. Bibette, 2011, Millifluidic droplet analyser for microbiology. Lab Chip, 11, 4057.

Liquid pearls

Liquid pearl

We report an experimental investigation on the formation of liquid core capsules having a thin hydrogel elastic membrane named liquid pearls. These fish-egg like structures are initially made of a millimetric liquid drop, aqueous or not, coated with an aqueous liquid film containing sodium alginate that gels once the double drop enters a calcium chloride bath. The creation of such pearls with micrometer thick membrane requires to suppress mixing until gelling takes place. Here, we show that superimposing a two dimensional surfactant precipitation at the interface confers a transient rigidity that can damp the shear induced instability at impact. Based on this, pearls containing almost any type of liquids can be created. This opens the possibility to use such structure as a new tool for screening microorganisms survival and growth in various three dimensional environment.

N. Bremond, E. Santanach-Carreras, L. Y. Chu, J. Bibette, 2010, Formation of liquid-core capsules having a thin hydrogel membrane: liquid pearls. Soft Matter, 6, 2484-2488.

N. Bremond, E. Santanach-Carreras, J. Bibette, 2011, Liquid pearls. Phys. Fluids, 23, 091108 (Gallery of fluid motion).