Motion of Nanodroplets near Chemical Heterogeneities

Abstract

We investigate the dynamics of nanoscale droplets in the vicinity of chemical steps which separate parts of a substrate with different wettabilities. Due to long-ranged dispersion forces, nanodroplets positioned on one side of the step perceive the different character of the other side even at a finite distance from the step, leading to a dynamic response. The direction of the ensuing motion of such droplets depends not only on the difference between the equilibrium contact angles on these two parts but in particular on the difference between the corresponding Hamaker constants. Therefore, the motion is not necessarily directed toward the more wettable side and can also be different from that of droplets which span the step.

Size dependent motion of nanodroplets on chemical steps

“…Since the wetting properties ͑i.e., the equilibrium contact angles eq ͒ are determined by the interplay between both the long-and short-ranged interactions, the force resulting only from the long-ranged part is not necessarily directed toward the more wettable side. For flat droplets this effect is robust, i.e., independent of the detailed properties of the chemical step ͑in contrast to the phenomena involving the approach toward chemical steps 22 ͒ and it can be stronger as compared with this latter approach. For flat droplets this effect is robust, i.e., independent of the detailed properties of the chemical step ͑in contrast to the phenomena involving the approach toward chemical steps 22 ͒ and it can be stronger as compared with this latter approach.…”

“…5,6 In view of the richness of the corresponding wetting phenomena and in view of their importance, they have been intensively studied both experimentally 7-10 and theoretically. [11][12][13][14][15][16][17][18][19][20][21][22] All the above mentioned applications of wettability patterns rely on the fact that the fluid placed on such a substrate will actually move toward the more wettable part. 2,[4][5][6] However, recently we could show that nanodroplets near chemical steps can move in the opposite direction.…”

“…2,[4][5][6] However, recently we could show that nanodroplets near chemical steps can move in the opposite direction. 22 Due to the long range of dispersion forces of the van der Waals type, droplets residing on one side of the step perceive the different character of the other side even at finite distances from the step and move accordingly. Since the wetting properties ͑i.e., the equilibrium contact angles eq ͒ are determined by the interplay between both the long-and short-ranged interactions, the force resulting only from the long-ranged part is not necessarily directed toward the more wettable side.…”

“…22 Due to the long range of dispersion forces of the van der Waals type, droplets residing on one side of the step perceive the different character of the other side even at finite distances from the step and move accordingly. 22 In the present study we show that, in contrast to a droplet only near a chemical step, the direction of motion of a droplet positioned on top of a chemical step can depend on the size of the droplet and that, depending on the substrate characteristics, sufficiently small droplets move toward the less wettable side. 22 In the present study we show that, in contrast to a droplet only near a chemical step, the direction of motion of a droplet positioned on top of a chemical step can depend on the size of the droplet and that, depending on the substrate characteristics, sufficiently small droplets move toward the less wettable side.…”

Dry-spot nucleation in thin liquid films on chemically patterned surfaces

“…Zhu et al (2010) fabricated binary surface topographies consisting of alternating ridges and droplet arrays by means of controllable dewetting. Koch et al (1995) studied the morphology and line tension of ultrathin liquid films in the vicinity of a chemical step heterogeneity using a density functional approach, whereas Moosavi et al (2008) investigated the motion of nanodroplets on similarly patterned substrates.…”