Electric fields enable tunable surfactant transport to microscale fluid interfaces

The transport dynamics of oil-soluble surfactants to oil-water interfaces are quantified using a custom-built electrified capillary microtensiometer platform. Dynamic interfacial tension measurements reveal that surfactant transport is enhanced under a dc electric field, due to electro-migration of charge carriers in the oil toward the interface. Notably, this enhancement can be precisely tuned by altering the field strength and temporal scheduling. We demonstrate electric fields as a new parameter to manipulate surfactant transport to microscale fluid-fluid interfaces.

Figure 1

Schematic of the experimental setup. Top left inset shows COMSOL simulations of electric field lines around a capillary of diameter 70 $\mu \mathrm{m}$.

Figure 2

Dynamic interfacial tension for (a) $4\mu \mathrm{M}$ OLOA 11000 in Isopar-M and (b) $10\mu \mathrm{M}$ Pluronic L64 in 100 cSt silicone oil. The dashed line represents the steady-state interfacial tension. Inset: Dynamic interfacial tension for each system during the last $1000\mathrm{s}$ under each electric field. The time axis has been shifted so that all the curves fit in the same time range of 0 to $1000\mathrm{s}$.

Figure 3

Time required for the oil-water interface to reach an interfacial tension of 35 mN/m for OLOA 11000 in Isopar-M and 30 mN/m for Pluronic L64 in silicone oil as a function of electric field. The dashed lines represent power-law scalings for OLOA in Isopar-M. The horizontal dotted line establishes that the timescale is independent of field strength for Pluronic in silicone oil. The filled symbols denote the timescale under zero electric field. Inset: Electrical conductivity as a function of surfactant concentration. The arrow shows the CMC of OLOA 11000 in Isopar-L [41]. The dashed line shows the linear dependence of conductivity with concentration for OLOA in Isopar-M. The dotted horizontal line shows that the conductivity of Pluronic in silicone oil is independent of surfactant concentration. The filled symbols denote the conductivity of pure oils, without externally added surfactant.

Figure 4

Dynamic interfacial tension for $4\mu \mathrm{M}$ OLOA 11000 in Isopar-M at zero electric field ($\circ$), $0.2$ kV/cm ($\diamond$), from zero field to $0.2$ kV/cm at $200\mathrm{s}$ ($△$), and from $0.2$ kV/cm to zero field at $100\mathrm{s}$ ($\square$). The arrows indicate the time when the field was switched off or on.