Uphill motion of droplets on tilted and vertical grooved substrates induced by a wettability gradient

• Micro-channels realized on a substrate could enhance the droplet displacement onto a horizontal surface.
• The influence of the substrate tilt on the droplet displacement is analyzed for smooth and patterned surfaces.
• A droplet could move uphill onto a vertical surface due to the presence of the micro-channels.

The spontaneous motion of droplets because of a gradient in surface energy presents many different potential applications (inkjet printers, pulsating heat pipes, etc.). Here, experiments were conducted on patterned silicon substrates. An etching step generated micro-channels with hydraulic diameters of 0.8 μm (height = 0.4 μm; width = 10 μm). The space between each micro-channel was 10 μm wide. The gradient in surface energy was created by photo-degrading a thin layer of octamethylcyclotetrasiloxane (OMCTSO) deposited onto the patterned substrate by plasma enhanced chemical vapor deposition (PECVD). The water contact angle of the photo-degraded OMCTSO was adjusted using time and photo-irradiation parameters. Deionized water was used as the working fluid and the volume of the studied droplets was 60 μL. The results show that the combination of surface energy gradient and etching improves the droplets displacement compared to smooth surfaces. The impact of gravity was also studied by tilting the substrates up to 20̊. For the horizontal cases the peak velocity reaches ∼60 mm/s for the patterned substrates (PS) (direction parallel to the micro-channels) and ∼64 mm/s for the smooth substrates (SS). Yet, the displacement reached 18 mm for the PS whereas it was 14 mm for the SS. For 20̊ tilted substrates, the peak velocity decreased to 10.2 mm/s for PS and to 22 mm/s for SS, while the displacement dropped to 9.6 mm for PS and to 7.6 mm for SS. By using this approach, droplets were shown to be able to move spontaneously uphill onto vertical surfaces (with micro-channels of height between 1 and 2 μm). The peak velocity reaches 7 mm/s for a total displacement of about 8 mm.

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