Choosing the best oil and surfactant for droplet generation

Choosing the best oil and surfactant for droplet generation

A widely used microfluidics approach to create emulsions is based on the production of droplets of one fluid dispersed in another immiscible fluid (mainly oils), at frequencies of the order of one to thousands of droplets per second. Droplet-based microfluidics utilizes the interaction between the interfacial tension and the fluidic shear force to break continuous fluids into uniform-sized segments within a microchannel. For this reason, the viscosity of oil can critically impact several parameters such as water-in-oil emulsion, and more precisely how droplet size and droplet generation rate are affected.

With the emergence of microfluidics, the market for oils dedicated to this field has grown considerably. The purpose of this review is to categorise the oils encountered in microfluidics according to their different physico-chemical characteristics and their impact on droplet parameters.

What is a surfactant?

Microfluidics is a process that generates thousands of drops per second, via a chip in which two immiscible fluids circulate in a network of microchannels, forming an emulsion. To stabilise these droplets, it is necessary to add a surfactant. A surfactant is an amphiphilic compound that lowers surface tension. It adsorbs naturally at interfaces because it is composed of a hydrophilic and a hydrophobic part. In this context, it is necessary to find oils compatible with the surfactant and which is compatible with the material used (for example cells).


Figure 1 : The surfactants stabilize the droplets by adsorbing to the oil–water interface, lowering its interfacial tension and coating the inner surface with a hydrophilic, non-ionic, and biocompatible layer, such as polyethylene glycol, that is resistant to protein adsorption and non-toxic to cells. Extrated and modified from Holtze C et al 2008 Lab Chip

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

Figure 2 : Analysis of the average droplet sizes by different viscosity and flow pressure conditions.
Discover our selection!


Explore our scalable solutions designed to meet the demands of high-throughput droplet generation with ease

Impact of oil viscosity on droplet generation rate

As shown in table 5 and Figure 3, Yao et al. 2019 shows that the number of droplets generated per unit time decreased as more viscous oil was used under all tested flow pressure conditions, where strong linear correlations were observed (R2 > 0.99 for all flow pressure conditions).
 Pw :Po (mbar) 5 cSt 7 cSt 10 cSt 15 cSt

30 :40

60 :80

90 :120

120 :160

150 :200

76 ± 1

157 ± 0

239 ± 8

275 ± 3

581 ± 12

66 ± 1

126 ± 2

215 ± 6

334 ± 6

499 ± 12 

54 ± 1

107 ± 3

182 ± 6

283 ± 3

411 ± 12

45 ± 0

93 ± 2

149 ± 1

223 ± 3

305 ± 8

Table 4 : Average droplet generation rate with different oil viscosity and flow pressure (µm) extracted from Yao et al. 2019 Micromachine.

Figure 2 : Analysis of the droplet generation rate by different viscosity and flow pressure conditions


The choice of the oil will essentially depend on your application. However, other parameters must be taken into account such as oil-surfactant and oil-material compatibility. Since the physical parameters of the continuous phase can influence the expected result, it is important to consider, for example, the viscosity of the oil.