Formation of droplet interface bilayers in a Teflon tube

Droplet-interface bilayers (DIBs) have applications in disciplines ranging from biology to computing. We present a method for forming them manually using a Teflon tube attached to a syringe pump; this method is simple enough it should be accessible to those without expertise in microfluidics. It exploits the properties of interfaces between three immiscible liquids, and uses fluid flow through the tube to pack together drops coated with lipid monolayers to create bilayers at points of contact. It is used to create functional nanopores in DIBs composed of phosphocholine using the protein α-hemolysin (αHL), to demonstrate osmotically-driven mass transfer of fluid across surfactant-based DIBs, and to create arrays of DIBs. The approach is scalable, and thousands of DIBs can be prepared using a robot in one hour; therefore, it is feasible to use it for high throughput applications.


Measurement of drop velocity and length
For Figure 1C; drop velocity was measured as detailed previously 32 using two orthogonal LEDs/photodiodes spaced 1 m apart along a transparent PTFE tube (343 µm bore; photodiodes were ~1.3 m from tube ends; Fig. S1). As all fluids used in a single experiment had different refractive indices, photodiode voltage varied depending on the fluid in the light path. Times taken by drops to travel through one light beam and between beams were recorded using custom software, with sampling frequency of up to 500 Hz and thereby provide negligible error of the average velocity of drops over the 1 m distance between photodiodes. It is important to determine tube diameter accurately since the film regions are very thin, so HFE7500 was pumped through a virgin tube at a known flow rate. The time taken for the leading HEF7500-air interface to travel between the two photodiodes was recorded, and average diameter calculated using the continuity equation and known flow rate set on the syringe pump. This must be done prior to any other fluids entering the tube as otherwise there may already be a film on the wall which would result in a lower tube diameter being calculated.

Drop velocity and film thickness
The thickness of a film engulfing a drop (as in Fig. 3) may be estimated by implementing the assumptions of an inviscid (µ 1 >> µ 2 ) or solid drop (µ 1 << µ 2 ), and applying continuity to the flow within a circular tube. For an inviscid drop, where R = channel radius, r = drop radius, h = film thickness, Q = flow rate, U = mean velocity of fluids, we obtain: The velocity in the film region is modelled as zero or a linear velocity profile (Couette flow) to determine the limits of the film thickness for the cases of inviscid or solid drops respectively (Fig. 3ii). Equating flow rates at axial positions in carrier fluid (phase 1) and drop region (phase 2) yields: These equations, or similar, have been derived by several authors for an inviscid 37,38 and solid drop. 32,39 Therefore, when flow is induced in the capillary illustrated in Fig.   S2, the distance between x f and x d reduces by ∆x which depends on film thickness.
The film thickness, in the limits of an inviscid or solid drop, may be estimated by measuring the velocities of the drop and carrier fluid. For all cases where µ 1 ~ µ 2 , it is expected to reside between these two limits. The same equations can then be applied to our new fluidic architecture illustrated in Figure 1A, where film thickness between water and oil phases may be estimated by measuring the mean velocity of both (assuming the carrier-fluid film surrounding the oil is unchanged over the length of the oil drop). The resultant film thicknesses, from the inviscid and solid drop equations, provide the limits of film thicknesses for any viscosity ratio between water and the engulfing oil drops.  indicates that pyranine is unable to diffuse through the oil from drop 2 to 1, and confirms that the formation of functional pores in Fig. 3.

Movies
Movie S1 2-D array of DIBs. This Movie illustrates the formation of the structure illustrated in Fig. 5vi. A train consisting of 3 coated water drops (containing yellow, blue, and red dyes) passes from the thin tube into the thick one. This results in the formation of 3 water drops engulfed in one oil drop; DIBs form at points where one coated water drop contacts another.

Movie S2
2-D array of DIBs. This Movie illustrates the formation of the structure illustrated in Fig. 5vii. A train consisting of 13 coated water drops (containing yellow, blue, or red dyes) passes from the thin tube into the thick one. This results in the formation of 13 water drops engulfed in one oil drop (water drops retain their original sequence); DIBs form at points where one coated water drop contacts another.

Movie S3
Compact array of DIBs. This Movie illustrates the formation of the structure illustrated in