2.1. Device fabrication and design
Standard photo lithography and soft lithography were used to fabricate the silicon mould and poly(dimethylsiloxane) (PDMS) microfluidic devices, respectively . To start with, a 4-inch silicon wafer was spin-coated with SU-8 3050 (MicroChem Corp.) photoresist which was followed by soft baking. Next, the photoresist was selectively exposed to UV light through a photomask. Subsequently, the unexposed photoresist was dissolved by the developer, leaving behind the mould with the channel patterns. Following the fabrication of the mould, soft lithography was employed to make the thin stretchable channel layers. Liquid PDMS (Sylgard 184, Dow Corning), prepared by mixing the base and curing agent at a 10: 1 ratio was spread on the mould to replicate the channels. The PDMS layer was subsequently cured for 2 hrs at 75°C. Next, two thick PDMS slabs were bonded to the inlet and outlet regions of the thin channel layer after plasma treatment (PDC-32G-2, Harrick Plasma). These slabs were used to hold the tubing connected to the chip. The inlet and outlet holes were then punched manually. Finally, the PDMS layer comprising the channel patterns with the thick slabs on it was plasma bonded to a plain PDMS layer.
Using the above steps, we designed and fabricated a stretchable micromixer with a curved serpentine mixing channel. The serpentine channel with 20 mixing units has a uniform width of 250 µm, and a height of 113 µm. The radius of curvatures is 312 µm. The total thickness of the stretchable microdevice is 1 mm in the mixing channel region and 5 mm at the inlet/outlet region.
2.2. Stretching platform
Fig. 2a illustrates the entire experimental setup for the stretchable micromixer. A custom-made stretching platform was designed and manufactured for automatic, unidirectional, periodic elongation of the micromixer, Fig. 2b. This platform is assembled on top of a poly (methyl methacrylate) (PMMA) sheet that can be fitted onto the stage of an inverted microscope for observation. A translation stage, driven by a hybrid stepper motor (JKM NEMA17 Two Phase 42 mm), is mounted on the platform for applying automated single-axis motion to the micromixer, Fig. 2b. This motor is controlled by a microcontroller board (Arduino), and can apply a periodic translational motion to dynamically stretch the micromixer along its length. The inlet and outlet areas of the stretchable device are firmly clamped by the PMMA holders so that no slip will happen during the stretching process, and the mixing unit (the channel section) can be stretched freely, Fig. 2c. The platform can stretch the micromixer along its length up to 4 mm with different frequencies at constant stretching velocity (triangle-wave displacement).
2.3. Experimental setup
The stretchable micromixer has two inlets for the introduction of the two fluids to be mixed. The first fluid is deionised (DI) water (Milli-Q® Direct 8 water purification system). The second is a 0.1% (w/v) solution of fluorescein sodium salt (Sigma-Aldrich) in DI water. The fluorescent solution and DI water were loaded in glass syringes and delivered into the microdevice using a flow-rate-controlled syringe pump (neMESYS, Centoni GmbH). We used a fluorescence inverted microscope (Nikon Eclipse Ti2) coupled with an LED illumination source (pE-4000, CoolLED) to visualise of the mixing behaviour of the fluorescent stream in the stretchable micromixer. A digital camera (Nikon, DS-Qi2) was mounted on the microscope for recording fluorescent images of mixing channels. The mixing behaviour was evaluated at the outlet region where the device oscillation does not impact the focusing plane of the camera.
2.4. Data analysis
The recorded images from both the experimental setup and the COMSOL simulations were analysed using MATLAB to evaluate the mixing performance quantitively. For this purpose, the mixing index (MI) of the fluorescent solution in water was calculated as :
Where N is the total number of discrete samples taken (total number of pixels); is the average of the normalised concentration intensity field; and is the normalised concentration intensity for the ith pixel. The value of mixing index varies from 0 for the unmixed condition (binary mixture) to 1 for complete uniform mixing. This formula was employed over all pixels of the selected frame. The reported mixing indexes in this paper have been measured based on one period of elongation.
2.5. Numerical modelling
We further studied the effect of periodic elongation on the fluid flow and mixing performance of the stretchable micromixer using numerical simulation. The three-dimensional (3D) numerical model of the stretchable serpentine microfluidic chip was formulated in COMSOL Multiphysics 5.6 software. The governing equations for the physics of solid deformation, Navier-Stokes equations for the incompressible fluid with no external forces, and the diffusion equation for mass transport were solved numerically.
The whole process of periodic elongation, fluid flow, and mixing were modelled with three different physics provided in COMSOL: Solid Mechanics, Single-Phase Laminar Flow, and Transport of Diluted Species. Moving Mesh was also applied to the model for the time-dependent simulation. Water, as the input flow, was considered as a Newtonian incompressible flow with the following properties: ρ = 1,000 kgm-3, μ = 10−3 kgs-1m-1. The concentration of the dye was set at 0 mol/m3 for the two side inlets and at 1 mol/m3 for the middle inlet. The diffusion coefficient of the fluorescent dye was set at 10–9 m2s−1. The boundary conditions were no slip and no flux for the walls and zero pressure at the outlet.