Microdroplet Operations in Polymeric Microtubes

Abstract

Microsystem technologies allow a plethora of operations to be achieved for microemulsion- and microdroplet-based assays, providing miniaturized, yet large-throughput capabilities to assist experimentation in analytical chemistry, biology, and synthetic biology. Many of such approaches have been implemented on-chip, using microfluidic and lab-on-a-chip technologies. However, the microfabrication of such devices relies on expensive equipment and time-consuming methods, thus hindering their uptake and use by many research laboratories where microfabrication expertise is not available. Here, we demonstrate how fundamental water-in-oil microdroplet operations, such as droplet trapping, merging, diluting, and splitting, can be obtained using straightforward, inexpensive, and manually fabricated polymeric microtube modules. The modules are based on creating an angled tubing interface at the interconnection between two polymeric microtubes. We have characterized how the geometry and fluid dynamic conditions at this interface enabled different droplet operations to be achieved in a versatile and functional manner. We envisage this approach to be an alternative solution to expensive and laborious microfabrication protocols for droplet microfluidic applications.

Figure 1

Experimental setup. W/O droplets were formed using an in-house-developed instrument. A syringe pump continuously aspirated water or oil volumes into a loading tubing (inner diameter 0.15 mm, outer diameter 0.4 mm) from a fluid container, where different aqueous solutions were placed in wells and covered by a common oil layer. A computer-controlled X–Y stage moved the container underneath the loading tube, so that the desired phase could be selected. Additionally, the loading tube was lowered in a time-controlled manner into the desired fluid, resulting in a W/O droplet of defined volume to be formed. Droplets were subsequently transported to the angled tubing interface for the desired operation. Droplet operations at the angled interface were monitored under a microscope or recorded with a color camera for further analysis. Scale bar is 600 μm.

Figure 2

Droplet trapping at an angled OT–IT interface. (A) 2D and 3D schematics showing the fabrication of angled tubing interface: the IT is made of either a tapered microtube with circular cross section (thin end of a gel-loading pipette tip) or a flattened tube with an elliptical cross section. The IT is cut to the desired angle and inserted into the OT. (B) Side view schematic (left column) and images (right column) of the three key steps to achieve droplet trapping at the angled interface (circular cross section or elliptical cross section). First, the droplet moves toward the interface with a constant velocity (i). When the droplet approaches the angled interface (ii), the frontend of the droplet is guided onto the sloped IT, while its backend remains in the OT. The droplet continues to move forward until a bypass gap for the oil phase (from point a to point b) is created into the IT. When a balance is reached between the Laplace pressure and the hydrodynamic pressure around the droplet, this eventually stops and remains trapped (iii). Scale bar is 600 μm. (C) Schematic representation of the side view and top view of a trapped droplet at the angled interface for both types of IT. The blue lines show different trajectories of the oil flow path from OT to IT around the droplet.

Figure 3

Condition to trap droplets at angled tubing interfaces. (A) Schematic illustrating the geometrical configuration of the maximum droplet volume that can be housed at the angled tubing interface according to the parameter α (derivation in the Supporting Information). (B, C) Plots showing the trapping efficiency of the angled tubing interface for three angles, α ∼5°, ∼20°, and ∼45°, with respect to the same range of flow rates. The largest volume of the trapped droplets was normalized to the theoretical value of a trapped droplet at the angled interface, given by (1/12 + cot α/8) × π × D_OT³. Regions under the curve represent conditions that enable droplet trapping.

Figure 4

An angled interface enables controlled droplet merging operations. (A) Schematic diagrams and snapshots that depict the key processes of droplet trapping, merging, and release. (i) The first droplet (green) is trapped, waiting for a second droplet (red) to reach the interface. The blue arrow indicates the flow direction. (ii) Two droplets are merged. (iii) The merged droplet moves toward the IT. (iv, v) The merged droplet travels into the IT and is released from the trap without breaking. (B) A representative example of the merging of three droplets (red, blue, and white). Scale bars are 600 μm.

Figure 5

An angled interface enables droplet dilutor operations. (A) Size of the input droplet (diluent) is identical with that of the output droplet (diluted) (n = 5). (B) Key steps of a droplet dilutor: (i) a mother droplet (red) is trapped at the angled interface waiting for diluent droplets (green) to coalesce. (ii) After a diluent droplet is merged with the mother droplet, the bypass gap is blocked. (iii) The frontend of the merged droplet is pushed into the IT. (iv) The merged droplet reduces its volume in the OT and Rayleigh–Plateau instability emerges around the neck in the IT. (v) The front tip of the merged droplet pinches off (v), leaving the mother droplet at the angled interface. Relying on the viscous shear induced by the bypass flow, the trapped mother droplet is thoroughly mixed after time (vi). The blue arrows indicate the direction of oil flow, while the black arrows indicate the inner droplet eddies. (C, D) A representative example of 15 droplets pinched off from the mother droplet containing originally calcein. The fluorescent intensity of each droplet is normalized to the value of the first droplet pinched off. Experiments were repeated three times. Scale bars are 600 μm.

Figure 6

An angled interface enables the splitting of plugs into daughter droplets. (A) Schematic of a plug splitting into smaller daughter droplets and image of a train of daughter droplets. (B) Characterization of size polydispersity of daughter droplets under different flow rates. When calculating polydispersity, the first two droplets were excluded. Typically, a plug of 0.6 μL could be split into ∼20 droplets. Higher flow rate enables smaller polydispersity of sizes of droplets. Each condition was repeated at least three times. Scale bar is 600 μm.