Fast Transport of Water Droplets over a Thermo-Switchable Surface Using Rewritable Wettability Gradient

Abstract

In spite of the reported temperature dependent tunability in wettability of poly(N-isopropylacrylamide) (PNIPAAm) surfaces for below and above lower critical solution temperature (32 °C), the transport of water droplets is inhibited by the large contact angle hysteresis. Herein, for the first time, we report on-demand, fast, and reconfigurable droplet manipulation over a PNIPAAm grafted structured polymer surface using temperature-induced wettability gradient. Our study reveals that the PNIPAAm grafted on intrinsically superhydrophobic surfaces exhibit hydrophilic nature with high contact angle hysteresis below 30 °C and superhydrophobic nature with ultralow contact angle hysteresis above 36 °C. The transition region between 30 and 36 °C is characterized by a large change in water contact angle (∼100°) with a concomitant change in contact angle hysteresis. By utilizing this "transport zone" wherein driving forces overcome the frictional forces, we demonstrate macroscopic transport of water drops with a maximum transport velocity of approximately 40 cm/s. The theoretical calculations on the force measurements concur with dominating behavior of driving forces across the transport zone. The tunability in transport velocity by varying the temperature gradient along the surface or the inclination angle of the surface (maximum angle of 15° with a reduced velocity 0.4 mm/s) is also elucidated. In addition, as a practical application, coalescence of water droplets is demonstrated by using the temperature controlled wettability gradient. The presented results are expected to provide new insights on the design and fabrication of smart multifunctional surfaces for applications such as biochemical analysis, self-cleaning, and microfluidics.

Thermo-Responsive Wettability via Surface Roughness Change on Polymer-Coated Titanate Nanorod Brushes Toward Fast and Multi-Directional Droplet Transport

“…The water droplet moved rapidly to the heated part at a velocity of ~18 mm/s. Although the transport velocity was lower than that reported on the PNIPAAm-grafted structured PDMS, 29 the velocity of ~18 mm/s observed on the Paraffin/TNR brush is still higher than those reported so far, such as spindle knot structure coated with PNIPAAm (about 0.1 μm/s), polymer-coated TiO2 nanorods (a few μm/s) and photoisomerizable monolayercoated SiO2 (about 1 mm/s). [35][36][37] The fast droplet transport is probably due to the fact that the large contact angle changes with a temperature difference of a few degrees at around 50 °C; a large wettability gradient force was generated by the large dcosθ/dx on a substrate with a temperature gradient.…”

“…For instance, the CAs on a flat surface coated with PNIPAAm change from ~63° to ~94° at higher temperature than LCST (around 32 °C). 29 The OTMS-modified TNR brush, that is uncoated with paraffin, also exhibited the same CA irrespective of substrate temperature. Consequently, the drastic change on wettability for the Paraffin/TNR brush is attributed to surface roughness change by heating/cooling, while the surface chemical properties remain unchanged.…”

Thermo-responsive wettability via surface roughness change on polymer-coated titanate nanorod brushes toward fast and multi-directional droplet transport

“…For instance, the CAs on a at surface coated with PNIPAAm change from $63 to $94 at higher temperature than LCST (around 32 C). 32 The OTMS-modied TNR brush, that is uncoated with paraffin, also exhibited the same CA irrespective of substrate temperature. Consequently, the drastic change of wettability for the paraffin/TNR brush is attributed to surface roughness change accompanied by phasetransition of paraffin by heating/cooling, while the surface chemical properties remain unchanged.…”

Thermo-Responsive Wettability via Surface Roughness Change on Polymer-Coated Titanate Nanorod Brushes Toward Fast and Multi-Directional Droplet Transport

“…dcosθ/dx represents the spatial gradient of wettability as θ and x represent contact angle and the length of the substrate. 17,29 The Paraffin/TNR brush exhibits higher hydrophobicity with a CA of ~157° at temperature below 50 °C (at room temperature), and at temperature above 50 °C the surface drastically changes to a less hydrophobic state with a CA of ~118°. In this system, a droplet transport can be achieved by a spatial heating of the Paraffin/TNR brush at room temperature as the temperature gradient applied on the substrate yields a drastic wettability gradient at the point with around 50 °C; the droplet moves toward spatially heating parts.…”