Electrostatically-induced trajectory switching system on a multi-inlet-multi-outlet superhydrophobic droplet guiding track

Background

Open channel microfluidic systems move small droplets for reactions and solution transfer, usually by gravity along hydrophilic patterns on superhydrophobic surfaces, but such patterns contaminate the track and cause water loss. Grooved superhydrophobic Cu tracks guide droplets without these losses, and electric methods can transport, sort or merge droplets, yet in the latter the droplets are exposed to the electric field throughout and must stay in contact with electrodes.

Design and approach

Shadow grooves 1.5 mm wide and 0.5 mm deep were milled into a 1 mm-thick Cu plate by computer numerical control (CNC). Ag nanostructures were grown on the grooves by electroless galvanic deposition in 3 mM AgNO3, and the surface was coated with 1H,1H,2H,2H-perfluorodecanethiol (PFDT) to form a PFDT Ag/Cu guiding track. Capacitors of Ti (50 nm) and Cu (300 nm) patterned on a glass slide were held 2 mm above the junction of a reversed Y-shaped groove; when one capacitor is biased, charges accumulate at its electrode edges and the resulting field pulls a passing droplet toward that side, while the droplet touches only the grounded superhydrophobic track.

How it was evaluated

The authors measured water contact angles (CAs) and sliding angles (SAs) as a function of galvanic reaction time, imaged the Ag nanostructures by field emission scanning electron microscopy, and recorded droplet motion on tracks tilted by 5°. The electrostatic field and force on the droplet at the junction were simulated in COMSOL Multiphysics 4.3, treating the droplet as a 1 mm-radius sphere. The x-directional electrostatic force (Fex) was estimated from the critical sliding angle on a line-patterned track at 0.4 kV to 1.8 kV for 3, 5 and 7 μL droplets and compared with simulation, and routing outcomes were mapped against substrate tilt and applied voltage before testing a MIMO track.

Key results

On flat PFDT-coated Cu, the water CA was 117.3° and the SA exceeded 90°. On the Ag/Cu surface, the CA saturated at 160° or above for reaction times longer than 20 min and the SA fell to about 1°; after 30 min of reaction, the Ag formed a double-roughness structure of 300 nm to 1 μm clusters and 100 to 200 nm particles, and the CA was 160° ± 1.23°. A 5 μL droplet rolled along the curved Y-shaped track at a tilt of 5° with a velocity of 5.71 cm s−1.

At the junction of the reversed Y-shaped track, a droplet was deflected toward whichever capacitor carried the 1 kV bias. In the simulation, the field on the droplet surface peaked at 0.59 MV m−1 and the electrostatic force at 8.21 μN when the droplet was closest to the anode, and the force fell as this distance increased. Because the droplet crossed the junction within 0.08 s and the field at the substrate surface was about 0.1 MV m−1, the authors considered electrowetting negligible. Canola oil droplets could not be steered, which the authors attribute to their non-polar character.

Below 0.7 kV the electrostatic force was too small to hold the droplet, and above this voltage the experimental Fex agreed with the simulated Fex within a relative error of 10.1%; 3 μL and 7 μL droplets gave sliding angles similar to 5 μL droplets. Droplets were misguided when Fg/Fex exceeded 2.93, trapped at the junction when it was below 1.04, and guided along the intended path between these values, with stable operation requiring a sliding angle below 15° and a voltage below 1.4 kV.

On a MIMO track with three inlets and four outlets, placed 2 mm below a glass slide carrying 12 capacitors and tilted by 5°, dyed 5 μL droplets were routed by selectively biasing capacitors at 1 kV: from inlet 1 to outlet 1, from inlet 3 to outlet 3, and from inlet 2 to either outlet 2 or outlet 4.

Open questions

The switching works only when the electrostatic force is balanced against the gravitational force on the droplet, which restricts operation to a window of tilt angle and applied voltage. Non-polar canola oil droplets were not deflected, so the method as reported applies to water droplets.

How it connects to other research

Slippery liquid-infused porous surfaces (SLIPS) lower droplet adhesion with an infused lubricant layer, whereas this track keeps droplets on a superhydrophobic Ag/Cu surface and changes their direction with an electrostatic force applied from a capacitor that does not touch them.