Choosing the best oil and surfactant for droplet generation

The different types of oils

Typically, mineral oils have been used with cells. For example, focusing on encapsulation mechanisms [Chaber M and Viovy JL. 2008 Proc Natl Acad Sci ; Um E et al. 2010 Apll Phys Lett], and also as a double bacteria emulsification system to screen with incubation times of a few minutes and limited to about 2 hours [Aharoni et al. 2005 Chem Biol]. Among mineral oils, we find hydrocarbon oils. They are therefore limited to the applications such as PCR where the objects of interest (the DNA or RNA fragments) do not exchange between the droplets. Fluorinated oils are the only examples where cell survival and proliferation have been demonstrated with long-term incubation of organisms [Schmitz CHJ et al. 2009 Lab Chip] where gas exchange become crucial. The choice of fluorinated oils is mainly driven by two points : first, they are appealing since most organic compounds are insoluble in these oils. The compounds encapsulated in the droplet should not phase partition and therefore remain in the droplet which solves the exchange limitations of organic and silicone oils. The second advantage of fluorinated oils is their biocompatibility [Baret C. 2012 Lab Chip]. In addition, fluorinated oils are able to solubilise gases, which is critical for cell viability. To a lesser extent, it is possible to use silicone oils but the incompatibility with PDMS means that they remain very little used in microfluidics.

Which application for which oil?

In the following tables you will find the oils required for each application and the associated surfactants. These tables have been extracted from Baret 2012 Lab Chip.

Silicon oil

Surfactant	DC200	PDMS	AR20
Triton X-100			PCR (complex oil mix)
SDS (in water)		Oil-in-water emulsification
ABIL EM90			Directed evolution
No surfactant	Chemical compound storage Emulsification	Raman measurement

Table 1 : corresponding applications for water-in-silicon oil emulsions in microfluidics and associated surfactant

Hydrocarbon oil

Surfactant	Hexadecane	Tetra/octa/dodecane	Mineral oil	Isopar M	Vegetable / organic
Span 80	Emulsification Coalescence Droplet splitting Interfacial instabilities Laser manipulation Electrocoalescence Droplet sorting	Emulsification Droplet patterns Chemical coupling	Protein expression Droplet patterns Molecular exchange Cell encapsulation Chemical reactions Electrically-assisted emulsification Electrowetting emulsification	Emulsification Droplet pairing Electrocoalescence
Monolein Oleic acid			Tip streaming	Bilayers (squalane)
Tween 20/80	Dynamic surface tension Emulsification
Synperonic PEF C12E8	Dynamic surface tension		Tip streaming
SDS	Dynamic surface tension Emulsification
n-butanol			Interfacial rheology
ABIL EM90			PCR in droplet Directed evolution
Phospholipid	Lipid bilayer
No surfactant			Droplet hydrodynamics PCR in droplets		Droplet hydrodynamics (sunflower oil)

Table 2 : Corresponding applications for water-in-hydrocarbon oil emulsions in microfluidics and associated

Florinated oil

Surfactant	PFH/PFC/PFD/PFPH	Carrier oil (Raindance Technologies)	HFE/Novec	FC40	FC70/FC77	FC3283
PF-octanol	Reaction kinetics Protein crystallisation			microPIV	Compound screening Cell sorting	Protein crystallisation
PF-decanol PF-TD acid				PCR in droplet		Protein adsorption
PF-TD OEG						Protein adsorption Chemistrode
PFPE- COOH			Splitting droplets Droplet detection	Electrocoalescence
PFPE- COONH4		Cells in droplet	Multiple empulsion	Cells in droplets Droplet hydrodynamics Chemical reaction
PFPE-PEG		Cells in droplet PCR Diagnostics	Directed evolution Picoinjector PCR and diagnostics DNA amplification Sorting micro-organims Yeasts in droplets	Cells in droplets Dropspot Coalescence Chemical gradients Droplet separation Electrocoalescence	Molecular exchange Chemical gradients
PFPE-DMP			Cells in droplets Coalescence	Bacteria in droplets
Short chains						Surface tension and emulsification
No surfactant			Bacteria/antbiotics
FluoSurf		Cells in droplet PCR Diagnostics	Water in oil droplet generation Encapsulation Alginate/Polyacrylate/Agarose/Epoxy beads 3D cell culture in droplet PCR and diagnostics DNA Single- cell analysis Cell sorting	Water in oil droplet generation Encapsulation Alginate/Polyacrylate/Agarose/Epoxy beads 3D cell culture in droplet PCR and diagnostics DNA Single- cell analysis Cell sorting

Table 3 : Corresponding applications for water-in-fluorinated oil emulsions in microfluidics and associated surfactant

Compatibility between oils and chip materials

The choice of oil according to the material of the microfluidic device is also a parameter not to be neglected. Indeed, the oil is in close contact with the walls of the channels and a chemical reaction between them could be critical for the expected final result (e.g. formation of droplets of a precise size). Thus, the change of material for the microfluidic system also requires a change in the oil used. Indeed, it is imperative that the oil correctly wets the walls in order to form droplets of water in the oil and not droplets of oil in the water. This short section will therefore allow you to choose the right oil for the materials used in the manufacture of your microfluidic devices.

Fluorinated oils

The easiest choice to make remains that of fluorinated oils, since they are compatible with most metals, plastics and elastomers, the best known of which is FC 40. They are also highly recommended in devices made of Teflon/PEEK, especially for droplet nucleation applications in order to get closer to homogeneous nucleation [Ildefonso et al. 2013 J. Am. Chem. Soc.]. In this case, it will be a question of using fluorinated oil of the FMS type.

Silicone oils

Of course, silicone oils are highly recommended for microfluidic devices made of silicone. They are also compatible with PDMS or a new material called COC for Cyclic olefin copolymer.

Mineral oils

Concerning mineral oils, very little information is available regarding their compatibility with microfluidic device materials. However, it has been reported that these oils can be used well with devices made of polycarbonate (PC).

Examples of oil influence on droplet parameters

Impact of oil viscosity on droplet size

In Yao et al. 2019, they used deionized water with red color dye and mineral oil (Zhanhong Chemical Industry, Guangzhou, China) mixed with a surfactant (Span-80, 7.5 g/L, Sigma-Aldrich, St. Louis, MI, USA) as the dispersed and continuous phase, respectively. Four different viscosities (5, 7, 10 and 15 cSt) of the mineral oils were tested to investigate how the different viscosities of the continuous phase affects droplet size and droplet generation rate in water-in-oil emulsion. As can be seen in Figure 2 and table 4 the droplet size decreased as the oil viscosity increased.

Pw :Po (mbar)	5 cSt	7 cSt	10 cSt	15 cSt
30 :40 60 :80 90 :120 120 :160 150 :200	68.3 ± 2.0 43.6 ± 2.1 37.1 ± 1.8 34.0 ± 1.9 32.1 ± 1.0	57.0 ± 1.7 39.5 ± 1.8 33.4 ± 2.0 30.7 ± 0.9 29.1 ± 1.1	51.0 ± 1.7 35.5 ± 1.7 31.0 ± 1.0 28.1 ± 1.0 26.9 ± 1.1	46.3 ± 1.8 32.2 ± 0.9 28.9 ± 0.9 26.0 ± 1.1 25.1 ± 1.3

Table 4 : Average diameter of droplets with different oil viscosity and flow pressure (µm) extracted from Yao et al. 2019 Micromachine