Growing drops on an inclined plate: Onset of sliding

Guest talk by Simeon Völkel and Kai Huang (University of Bayreuth)


The PhD colloquium on June 29, 2018 will begin with a guest talk by Simeon Völkel and Kai Huang (University of Bayreuth) who will present their experiment on depinning processes of drops on an inclined plate. The subsequent CRC talk „Merging drops with unstable wetting films“ will be held by Peyman Rostami (project A02).

Liquid drops sitting on or running down an inclined plane are ubiquitous in our daily lives. Their sliding can be triggered by tilting the surface at a fixed drop volume or by increasing the drop volume at a fixed inclination angle. A recent numerical investigation [1] revealed that the two triggering protocols lead to different depinning processes of the drops.

Simeon Völkel and Kai Huang address this phenomenon experimentally with a conventional inkjet printhead, which provides a volume resolution of 22 picoliters and high repeatability [2]. By selecting different nozzles in realtime, we are able to follow the movement of the drop center precisely.

A detailed analysis of the drop shape will be presented. Völkel and Huang compare their results to numerical simulations in the quasi-static limit and find good agreement. In addition, they analyze the evolution of the contact angle during the drop's growth. Continuous volume growth leads to intermittent pinning and de-pinning transitions of the three phase contact line.

Inhomogenities of the surface lead to clear fluctuations of advancing and receding contact angles and the contour of the drop. Possible explanations for the fluctuations from the perspective of self-organized criticality [3] will be discussed.

[1] Semprebon and Brinkmann, Soft Matter, 10, 3325 (2014)
[2] Völkel and Huang, EPJ Web of Conferences, 140, 09035 (2017)
[3] Jensen, 'Self-Organized Criticality: Emergent Complex Behavior in Physical and Biological Systems' (1998)

Subsequent CRC talk:

Merging drops with unstable wetting films
Peyman Rostami (project A02)

Fusion or merging of drops plays a key role in many different processes such as raining and coating. Due to this fact, in recent years, it has been widely studied experimentally, theoretically and numerically. Physics of fluids stipulates that after coming in contact, two drops merge to reduce interfacial energy. However the pathway might be complex [1], [2]. When two immiscible drops merge, depending on the surface and interfacial tensions, a liquid film of the drop with lower surface tension might be drawn over the surface of the other drop, i.e. the spreading coefficient can be positive.

In a current study, the drops are deposited on a glass substrate via two independent syringes. We present results for pure water and cyclohexyl bromide. First a water droplet is deposited on the substrate and then cyclohexyl bromide is injected as the second drop. A thin liquid film of cyclohexyl bromide covers the water droplet surface.

In our case, these liquid films exhibit instability that resembles the Rayleigh–Plateau instability (Fig. 1), which is responsible of breakup and atomization of liquid sheets [3]. We present a detailed study of this instability analyzing the onset of instability, the length of liquid film and characteristic wave length of instability as function of physical parameters. The results show that the instability only occurs when a cyclohexyl bromide drop comes to contact with pre-deposited water drop. If the cyclohexyl bromide drop is deposited first, no instability is observed. However the final configuration of both processes is same, the pathway is totally different.

[1] S. Karpitschka and H. Riegler, „Noncoalescence of Sessile Drops from Different but Miscible Liquids: Hydrodynamic Analysis of the Twin Drop Contour as a Self-Stabilizing Traveling Wave,“ Phys. Rev. Lett. 109, (2012), 066103
[2] S. Karpitschka and H. Riegler, „Sharp transition between coalescence and non-coalescence of sessile drops,“ J. Fluid Mech. 743, (2014).
[3] R. Krechetnikov, „Stability of liquid sheet edges,“ Phys. Fluids. 22, (2010), 092101.

Date: June 29, 2018, 11:00-12:00
Venue: L2|06, Room 100

A cyclohexyl bromide drop (right) in contact to a water drop (left). A positive spreading coefficient pulls a thin film of cyclyhexyl bromide over the water drop. This film however becomes unstable and disintegrates into drops. Image: Peyman Rostami