Spreading dynamics on lithium niobate: An example of an intrinsically charged ferroelectric surface

Abstract

Droplet wetting and manipulation are essential for the efficient functioning of many applications, ranging from microfluidic applications to electronic devices, agriculture, medical diagnosis, etc. As a means of manipulating droplet wetting, the effect of applying an external voltage or surface charge has been extensively exploited and is known as electrowetting. However, there also exist many materials which bear a quasi-permanent surface charge, like electrets, which are widely employed in sensors or energy storage. In addition, other materials in nature can acquire surface charge by the triboelectric effect, like human hair, natural rubber, and polymers. Nevertheless, there do not exist any studies on spreading on this class of charged surfaces. In our work, we for the first time investigate spreading dynamics on lithium niobate (LiNbO3) as an example of a ferroelectric material with strong instantaneous polarization (0.7C/m2). We find a spreading behavior that significantly differs from classic surfaces. Spreading times can be significantly enlarged compared to standard surfaces, up to hundreds of seconds. Furthermore, the classic Tanner’s law does not describe the spreading dynamics. Instead, the evolution of the droplet radius is dominated by an exponential law. Contact angles and spreading dynamics are also polarization-dependent. They are also influenced by adsorption layers, such as they are left behind by cleaning. Overall, all results indicate that adsorption layers play a significant role in the wetting dynamics of lithium niobate and possibly other charged materials, where such processes are very pronounced. Possible mechanisms are discussed. Our findings are essential for the understanding of wetting on charged surfaces like ferroelectric materials in general. The knowledge of surface charge-based wettability difference, surface charge specific adsorption and its impact on wettability can be utilized in applications like, printing, microfluidics, triboelectric nanogenerators, and to develop biocompatible components for tissue engineering.