Guided Transport of Water Droplets on Superhydrophobic–Hydrophilic Patterned Si Nanowires

Background

Hydrophilic patterns on superhydrophobic surfaces are used in open-channel microfluidics, water harvesting, biomolecule analysis and droplet transfer. Earlier routes to such patterns, including mussel-inspired dopamine coatings, UV photocatalysis of titanium dioxide (TiO2) nanostructures, galvanic displacement, photopatternable polymer mixtures and lithography with plasma treatment, require additional complex processing, and the authors note that they are not well suited to controlling droplet direction in combination with Si-based electronics.

Design and approach

Vertical Si nanowire arrays are etched into p-type Si (100) wafers by aqueous electroless etching (AEE) in 30 mM AgNO3 and 4.9 M HF at 80 °C, with the nanowire length set by the etching time. The arrays are made superhydrophobic by immersion in 3 mM DTS in hexane for 30 min and baking at 150 °C for 1 h. Exposure to a 75 W mercury lamp (185 and 254 nm, 500 μW/cm², 3 cm working distance) decomposes the DTS alkyl chains through OH radicals and atomic oxygen generated from ozone, which makes the exposed nanowires hydrophilic; shadow masks and a 3 h exposure define the hydrophilic tracks. Because the surface is superhydrophobic in the Cassie state, with air trapped between the droplet and the bundled nanowire tips, the wettability contrast between track and surroundings confines a moving droplet to the track.

How it was evaluated

Advancing, receding and static water contact angles (CAs) were measured with about 7.5 μL droplets as functions of the etching time and the UV illumination time (averaged over five locations for the etching-time series), and the decomposition was followed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Droplet transport was recorded on tracks tilted at 5°, the critical droplet volume for transport was measured for track widths from 350 to 600 μm on five samples per width, and DTS-coated flat Si was processed in the same way as a reference.

Key results

The nanowire length increased from 0.2 to 14.7 μm as the etching time increased from 2 to 30 min, and nanowires etched for 30 min had diameters of 100 to 150 nm. On DTS-coated flat Si, the advancing and receding CAs were 109.3° and 92.6°. On the nanowire arrays, the CAs rose sharply during the first 3 min of etching and saturated at 150° or more for etching times longer than 10 min; droplets then rolled off at tilt angles below 5°. The abstract gives the CA hysteresis as less than 7°, whereas the Results section states less than 10° for surfaces with CAs above 150°.

UV illumination reduced the static CA of about 15 μm-long DTS-coated nanowire arrays from 158° to below 5° within 180 min, after which it remained constant. XPS showed a decreasing C 1s peak at 285.1 eV, assigned to C−C bonds of the alkyl chains, and increasing O 1s (532.3 eV) and Si 2p (103.1 eV) peaks associated with Si−O bonds; SEM showed DTS residues only at the bundled tips and no significant change on the nanowire sides. With a shadow mask, the exposed half of a substrate became hydrophilic while the covered half stayed superhydrophobic, and repeated cycles of coating and UV exposure produced no significant change in the CAs.

On a 500 μm-wide track, the droplet velocity was 6.91 cm/s, compared with 16.54 cm/s on the unpatterned surface, which the authors attribute to the increased interaction between droplet and track; they report that this is more than 3 times faster than transport in earlier studies based on surface energy gradients on flat surfaces. After recoating with DTS, droplets moved in straight lines under gravity, and a new mirror-image track defined by a second UV exposure guided them along the new path. Cross-linked tracks also transported a CdSe nanoparticle suspension, coffee and rat blood.

From the Young−Dupre relation, the authors calculated an interaction force of about 37 nN for an 8 μL droplet on the unpatterned surface (surface tension 72.8 mN/m, CA 158°, contact radius 1.5 mm) and, adding the adhesion force between track and liquid, an overall interaction force of 253.7 nN on the 500 μm track, where the CA perpendicular to the line decreased to 57°. The critical volume needed to move a droplet in a spherical shape increased linearly with track width between 350 and 600 μm. On DTS-coated flat Si, UV exposure lowered the CA from 109.3° to 10.1° within 70 min, but an 8 μL droplet stuck to the surface even at a tilt of 40° and was not transported along the tracks.

Open questions

The paper reports two operating conditions for transport: when the DTS was not fully decomposed (UV illumination shorter than 150 min), droplets stuck at defects on curved tracks, and droplets smaller than the critical volume wetted and spread along the track instead of moving.

How it connects to other research

Chaudhury and Whitesides moved droplets on a flat surface along a gradient in surface free energy; in this study, gravity on a tilted substrate drives the droplet and a hydrophilic track on a superhydrophobic nanowire surface confines it laterally.