Experimental method

– Different phases

In the previous study, we have shown that in the direct emulsion, fluorinated oil in water with 2%wt SDS as surfactant presents an adhesive behavior. This formulation allows to produce easily clusters with our dedicated devices. We would like to expand our study of the hydrodynamic assisted clustering to the inverted emulsions, i.e. the

1. In the presence of larger amount of micelles, the total force between surfaces may oscillate due to occurrence of oscillatory forces. Structural forces are a consequence of variation in the density of packing of small particles around a surface on the approach of a second one(stratification) [105].

Figure 5.3: Energy of adhesion between Span 80 monolayers in a silicone oil/dodecane mixture. The arrow indicates the insolubility threshold of the amphiphile[252].

water-in-oil systems. However, the swelling of untreated PDMS in the presence of certain solvents/oils require particular attentions, especially for the shallow channels [253]. In our study, we restricted our work to the use of mineral oil and fluorinated oil to ensure that no undesirable swelling effect occurs. Nevertheless, the swelling of the device could be avoided by various treatments of PDMS surface or by the use of other materials such as glass and microfluidic sticker etc. Therefore various organic solvents are potentially applicable, but this part is out of the scope of the present work.

– Surfactants

Two kinds of surfactant are used for the oil-in-water emulsion: anionic surfactant SDS and non-ionic polymeric surfactant Pluronic F-68. Sodium dodecyl sulfate (SDS) is a low-molecular mass organic compound with the formula CH₃(CH₂)₁₁OSO₃N a. It is the most widely used of the anionic alkyl sulfate surfactants. This surfactant is of an organosulfate consisting of a 12-carbon tail attached to a sulfate group, giving the material the amphiphilic properties. The class of Pluronic block is a widely used non-ionic surfactant which is a copolymer of hydrophilic ethylene oxide (EO) and hydrophobic propylene oxide (PO) arranged in A-B-A tri-block structure, the Pluronic F-68 has an average formula of (EO)₇₅−(P O)₃₀−(EO)₇₅.

For water-in-oil emulsions, we use the non-ionic surfactant sorbitan monooleate (Span 80) to stabilize water in mineral oil. With the formulaC₂₄H₄₄O₆, the value of hydrophilic-lypohilic balance (HLB)² is 4.3, rendering it more soluble in oil phase than in the aqueous phase. In order to stabilize the system of water-in-fluorinated oil, we use PEG di-Krytox, a copolymers of the A-B-A type with a (P O)₂ −(EO)₁₈ −(P O)₂ hydrophilic part in between the two perfluorinated chains.

The chemical structures of the four molecules are shown in the figure 5.4 and the details of physicochemical characteristics are listed in the table 5.1:

– Electrolyte solutions

Solutions of various concentrations of sodium chloride (NaCl) and potassium chloride

2. Hydrophile-Lipophile Balance is an empirical expression for the relationship of the hydrophilic and hydrophobic groups of a surfactant. The higher the HLB value, the more water-soluble the surfactant [254][255].

5.2. Sticky clusters 109

Figure 5.4: Chemical structures of the surfactants used.

Surfactant Type M_w(g/mol) HLB Emulsifier type CMC

SDS anionic 288.38 40 O/W 8.2mM(w/w)

Pluronic F-68 nonionic 8400 29 O/W 0.04mM

Span 80 non-ionic 428.60 4.3 W/O 0.025%

PEG di-krytox nonionic 14000 1.45 W/O 0.033%(w/w)

Table 5.1: Properties of surfactants used.

(KCl) are prepared. The presence of salt above certain quantity may induce the precip-itation of anionic surfactant SDS at room temperature. As a matter of fact, it is well recognized that SDS is very sensitive to the polarity of the medium. The solubility of the surfactants with sulfate group increases from the low hydrated large counterion towards the strong hydrated small counterions: K⁺ < N a⁺ < Li⁺. The order of this series is inverse for the carboxylate polar group[256][257] [258]. As for the non-ionic surfactant Pluronic F-68, the precipitation concentration value in the presence of salt is much higher due to the lack of charge.

Experiments

It is not obvious to characterize the aggregation process and the adhesion energy of the clusters obtained in our system. For example, we have thought to use the Dynamic Light Scattering (DLS) technique to probe the behavior of the emulsions. However, the smallest size of droplets is a few microns in diameter which reaches the limit of the light scattering technique. Furthermore, the fluorinated oil use is relatively heavy (ρ=1820 kg/m²) so that the sedimentation of droplets is not negligible.

We thus investigate the adhesion behavior of droplets directly within the microfluidic device. A qualitative analysis is given by the observation of the cluster formation in the microfluidic reservoir near the 3D step.

We use pressure control to generate the droplets with typical dilution phase pressure below 10 mbar. Working at low pressure is to ensure the droplets production under slow flow field so that we could better screen whether the adhesion takes place or not.

For the study of the surfactant effect, we use normal injection method in which

the continuous phase and dilution phase are exactly the same fluids, thus there is no concentration gradient across the channel. Regarding the study of the electrolyte effect, we use another method: the salt solution is injected only into the dilution channels. The continuous phase with surfactants as well as the dispersed oil phase are coming from the middle inlet as usual. With this coflow diffusion strategy, we are able to study systematically the effect of salt concentrations and salt types on the aggregation process.

The sketch of the two methods is shown in the figure 5.5. Finally, to produce the direct emulsion oil-in-water, we prepare hydrohylic PDMS system, and hydrophobic PDMS system for inverted emulsion water-in-oil.

Figure 5.5: Schematic drawing of two injection strategies. (a) the normal injection. (b) the coflow diffusion injection.