Self‐Propelling and Positioning of Droplets Using Continuous Topography Gradient Surface

Abstract

A radial pattern with continuous topography gradient is presented, which induces a continuous inward wettability gradient and enables self-propelling and accurate positioning of droplets to the pattern center. The effect of droplet size and wettability gradient of the pattern on the self-mobility of droplets is investigated. The wettability gradient is found to increase towards the pattern center, enhancing the self-motion of droplets at the inner area of the pattern. Moreover, larger droplets give rise to a larger solid-liquid contact diameter, which helps to satisfy the self-motion criteria that the advancing contact angle at front edge is smaller than the receding contact angle at rear edge. Consequently, a larger droplet size favors self-motion initiated from the outer area of the pattern. The continuous topography gradient employed here allows the flexible dispensing of droplets at any place within a certain range, and avoids potential pinning defects to droplets at geometrical discontinuities.An average self-motion velocity up to 4.0 cm/s for microliter-sized droplets is achieved on the resultant patterned surface.

Long-range spontaneous droplet self-propulsion on wettability gradient surfaces

“…The top image shows that the real shape of droplets along the gradient direction, the shape of droplets changed from spherical segment to thin film while the static CA changed from 166.0° to 15.5° accordingly. Unlike other wettability gradient surfaces with a range less than 120° 37, 43, 48 , the superhydrophobic and hydrophilic region on two ends greatly enlarged the wetting CA gradient range over 150°.…”

“…The grading of the chemical functional group density was recently extended to 2-D materials such as graphene, which can drive the droplet to move a total of 1–3 mm in one direction 25 . The motion of liquid droplets on chemically defined radial wettability gradients consisting of alternating wettability, i.e., hydrophilic and hydrophobic was also reported 42, 43 . Despite extensive progress, chemical gradient surfaces involve the functionalization of the surface with molecular gradient of alkanethiolate or alkylsilane, which may deteriorate due to migration or degradation of organic molecules and result in decay of chemical gradient in the long term operation.…”

“…While the hydrophobic coating provides a barrier for fabricating wettability gradient ranges from hydrophilicity to superhydrophobicity. As a result, the wettability gradient range based on the hydrophobic coating ranges from 100° to 150° 43 , which dramatically limits the droplet transport distance. Thus, to enlarge the wettability gradient, an intrinsically superhydrophilic surface is desired, however, the viscous energy dissipation imposed on such a design is large.…”

Directional imbibition on a chemically patterned silicon micropillar array

“…Anisotropic non-wetting surfaces are characterized by low adhesion and the Cassie Baxter state 9 with limited but anisotropic interaction between the droplet and the solid. Examples include anisotropic roll-off of water droplets from surfaces, 10 directional water droplet movement on topography gradients 11 and underwater anisotropic roll-off of oil droplets. 8 Surfaces that exhibit anisotropic wetting utilize higher surface energy coatings and most commonly the Wenzel state.…”

Tunable Contact Angle Hysteresis for Component Placement on Stretchable Superhydrophobic Surfaces

“…The wetting properties of a solid surface in contact with a liquid have a direct impact in numerous advanced technologies including flexible [1] and stretchable [2] electronics, in all application of bioinspired wettable surfaces [3,4] such as surfaces with superhydrophobicity/omniphobicity, [5][6][7][8][9][10][11][12][13] surfaces with patterned wettability, [14][15][16][17][18][19] integrated multifunctional surfaces and devices. [20,21] Increasingly, surface properties have been used to overcome the challenge of placing small components on flexible substrates by using, for example, adhesion [22,23] or and one for the deposition sites.…”