Focal Plane Shift Imaging for the Analysis of Dynamic Wetting Processes

Area of Science:

• Fluid dynamics
• Surface science
• Microscale phenomena

Background:

• Droplet-surface interactions are crucial in natural and industrial processes, with droplets exhibiting enhanced transport properties due to their high surface-to-volume ratio.
• Coalescence-induced droplet jumping on superhydrophobic surfaces is promising for applications like heat transfer and self-cleaning, but its dynamics are not fully understood.
• Existing imaging techniques struggle to capture the full three-dimensional (3D) motion of jumping droplets.

Purpose of the Study:

• To develop and validate a novel single-camera imaging technique for capturing 3D droplet motion.
• To investigate the fundamental mechanisms governing droplet jumping on superhydrophobic surfaces.
• To analyze the influence of various parameters on droplet jumping speed and trajectory.

Main Methods:

• Development of focal plane shift imaging (FPSI), a single-camera technique using focal plane manipulation to acquire 3D information.
• Application of FPSI to study droplet jumping on superhydrophobic surfaces with varying structure scales (10 nm to 1 μm) and droplet radii (3 μm to 160 μm).
• Benchmarking the FPSI technique and analyzing effects of droplet mismatch, multidroplet coalescence, and multihop coalescence.

Main Results:

• FPSI successfully resolved the full 3D trajectory of multiple jumping events.
• The study found that droplet departure angle is not dependent on droplet mismatch or the number of coalescing droplets.
• Angular deviation in droplet jumping is primarily caused by in-plane motion influenced by droplet pinning post-coalescence.

Conclusions:

• Focal plane shift imaging provides a powerful new platform for studying dynamic droplet processes involving out-of-plane motion.
• The findings challenge previous theories by identifying droplet pinning as the key factor controlling departure angle in droplet jumping.
• This research enhances fundamental understanding of droplet jumping and offers advanced imaging capabilities for related phenomena.