Bubble shadows add significant new dimension to film thickness measurement methodology

The impact of droplets onto smooth solid surfaces is a significant field of inquiry due to its occurrence in many industrial processes, from inkjets to fuel combustion to painting technology that employs sprays. In all these processes, the droplet impact plays a role as it forms a radially spreading lamella, bound within a bulging perimeter.

It can be challenging, however, to measure film thickness in these rapid processes with high-speed cameras from a lateral or top perspective, because “the forming ring at the outside obscures the thinner lamella within,” said co-author Patrick Foltyn. “Think of observing a pizza horizontally on a table. The bulging outer rim of crust makes it impossible to see the toppings in the middle.”

In 2019, Foltyn et al. determined the best measurement approach is from below and developed the laser pattern shift method (LPSM), a technique typically involving a prism to gauge internal light reflection at interfaces.

In their latest research, the scientists demonstrate an extended approach that adds a measurement principle based on shifts of known patterns within droplets through naturally occurring bubble shadows. The approach is “mostly a matter of existing data evaluation, so no new costly sensors are necessary,” said Foltyn.

The technique represents a simple and cheap method for accurately measuring a relatively complicated dimension. Its internal reflection configuration for droplet impact observations helps distinguish wetted areas from nonwetted areas, revealed as a significant factor in determining overall lamella thickness.

Immediate potential industrial applications of the technique include float glass production and lubricant film monitoring, among others. The method may help facilitate further research in fluid dynamics.

Source: “Measurement of the lamella thickness during droplet impact onto differently wettable smooth surfaces using an extension of the LASER Pattern Shift Method with naturally occurring patterns,” by P. Foltyn, L. K. Rihm, D. Ribeiro, A. Silva, and B. Weigand, Review of Scientific Instruments (2021). The article can be accessed at https://doi.org/10.1063/5.0060407 .