Focal Plane Shift Imaging for the Analysis of Dynamic Wetting Processes

Previous techniques used to observe droplet jumping come with challenges such as the inability to observe 3D droplet trajectories, key to understanding factors that affect departure angle and govern jumping speed and direction.

Researchers led by Dr. Nenad Miljkovic at the University of Illinois at Urbana-Champaign have developed a simple single-camera optical imaging technique to study the 3D trajectory of droplet jumping through focal plane manipulation.

The focal plane shift imaging technique affords 3D information during the study of coalescence-induced droplet jumping on superhydrophobic surfaces over a wide range of structure length scales and droplet jumping radii. The study is now published in the peer-reviewed journal ACS Nano.

The focal plane shift imaging technique used by the authors was effective in measuring the jumping droplet speed, polar and azimuthal angles of departure, which were all correlated with preceding research to validate its effectiveness.

Outcomes acquired from the focal plane shift imaging technique with two-droplet coalescence showed that both experimental results and numerical simulations correlated well with the theoretical inertial-capillary scaled velocity. For two-droplet coalescence, departure speeds reduced when mismatch in radius of the two initial droplets exceeded 40% due to reduced surface-to-kinetic energy conversion.

For three-droplet coalescence, an enhancement of jumping speed was discovered for a wide range of departure radii (3µm < R < 150µm) which showed that the capillary-to-inertial energy conversion mechanism can be successfully used for three-droplet coalescence.

For droplet coalescence involving more than three droplets, lower jumping speeds compared to theory indicated that the serial nature of multi-droplet coalescence was at play. This result indicates that droplet jumping might not occur for more than three droplet coalescence.

When observing the jumping droplet angle with the focal plane shift imaging technique, it was discovered that droplet size mismatch prior to coalescence does not play a direct role on angular deviation. For two-droplet coalescence having radii mismatch greater than 40%, the jumping polar angle greater than 80% for the majority of the events. For three- and multi-droplet coalescence, the experimental polar angle data showed larger scatter as compared to two-droplet coalescence with little correlation to radius mismatch.

The azimuthal angle of droplet jumping was also found to be independent of both radii mismatch and the jumping droplet radius, showing that the majority of jumping events occurred at azimuthal angles less than 30⁰ or greater than 150⁰.

The authors also indicated that droplet pinning was responsible for the random nature of large polar angle deviation during two-droplet coalescence. Non-generation of net momentum due to Laplace pressure was noticed in the case of non-equal radii droplets coalescing on a zero adhesion surface which had been additionally ideally discovered in two-droplet coalescence with large radii mismatch less than 40%. The authors attributed this to no prevalence of pinning at the contact line.

The focal plane shifting imaging technique implemented by the authors creates an avenue for extensive study of dynamic wetting processes which can be of relevance to future technologies in need of droplet motion.