Research: How spontaneous charging influences the motion of drops on a tilted surface

In April 2022, authors Xiaomei Li, from the Max Planck Institute for Polymer Research in Mainz, Germany, et al. have published an article with the title Spontaneous charging affects the motion of sliding drops in Nature Physics. They have found that the drop motion on a tilted surface is not only determined by viscous dissipation and activated processes, but also by electrostatic forces.

So far, the literature in the subject states that the motion of a liquid drop on a tilted surface is influenced by two factors: the viscous dissipation, which is due to hydrodynamic flow within the drop, and activated processes, i.e. the contact line overcoming energy barriers, resulting in contact-line friction. However, those two factors alone cannot predict the drop motion accurately; therefore, at least one other factor needs to be considered. The authors of this study believe that the missing factor is the electrostatic force

To prove their assumption, they first measured hydrophobic surfaces using the needle-in-drop-method on a sessile drop and chose surfaces with very similar contact angles for further experiments. The samples were surfaces coated in perfluorodecyltrichlorosilane (PFOTS), in detail: 1) silicon wafer with approx. 2 nm oxide layer, 2) 1-mm-thick SiO₂ coating , 3) 5-mm-thick SiO₂ coating. However, they differed in their substrate conductivities and in the thickness of the substrate. Sample 1) is a high-permittivity substrate with a negligible electrostatic force, sample 2) and 3) are low-permittivity substrates.

When the surfaces where tilted and drops rolled off them, the drop's paths differed on each sample. On sample 1), the velocity was high and increased in a linear fashion. This trajectory stayed constant, regardless if one or more drops where dosed on the same surface stretch. On sample 2) and sample 3), the first and second drop also showed a linear progression, although at lower velocities. However, the 100^th drop on both 2) and 3) showed a different velocity curve: those drops first moved much faster than the 1^st and 2^nd drops and then became slower in the second half of the measured slide length (=4.5 cm).

The authors explain those results with the electrostatic force at play between the drop and the surface. Literature has shown that on hydrophobic surfaces, drops in motion deposit negative electric charges on the solid surface - more so at the beginning of their path and ever less as they become more positively charged. As ever more drops leave behind negative charge, especially in the first part of their path, the electrostatic retardation decreases and thus additional drops begin to slide faster in this first part. When they reach the later part of the slide, where less negative electric charge has been deposited by their predecessors, they slow down as the retarding electrostatic force increases and they themselves deposit negative charge onto the surface.

In summary, the experiments discussed in the article give a good indication that the electrostatic force is indeed a third factor determining the velocity of moving drops. This research opens new possibilities for improving the control of drop motion in many areas, like printing, coating, and water management. Link to article 

We are proud that one of our contact angle meters of the OCA series was used to conduct the needle-in-drop-method and determine the advancing and receding contact angles of the sessile drops. The entire publication can be read on the journal's website, following the link below.