We’re presenting a drone equipped with an optical sensor for spray measurements—what we call a Spray Drone. Manual navigation remains a challenge, but we're actively developing AI-based software for autonomous drone positioning.

Spray scan in a painting process

In this project, we investigate a novel measurement method for detecting the color of droplets in a spray.

ParticleTensorAI® v.2.0.0 [Evaporating Droplet in an Acustic Levitator] #spray #droplets

You can create a liquid jet by pressing a liquid through a tiny pinhole of about 100µm or less. By adjusting the pressure correctly, you can produce a stable liquid jet that eventually breaks up in a chaotic regime.

To generate a droplet chain, a periodic disturbance must be introduced into the breakup process of the liquid. This is typically achieved using ultrasonic vibrations generated by a piezoelectric element. For water, the frequency is around 40kHz. In some studies, the disturbance has also been induced using a pulsed laser source.

By applying a periodic disturbance, it becomes possible to achieve a periodic breakup process, which leads to a droplet chain. The droplet size depends on the pinhole diameter. A good estimation is that the droplet diameter is roughly twice the pinhole diameter.

By adjusting the pressure, frequency, and pinhole size, you can create droplet chains that include satellite droplets, resulting in chains with two different droplet sizes. This is particularly useful for testing particle measurement devices such as Laser Diffraction (LDT), Phase Doppler (PDA) or Time-Shift (TSTOF). In this scenario, the calibration process covers a wide bandwidth.

In this video you have droplets sizes of 40µm and 150µm.

ParticleTensorAI® with Thermally Insulated Window for Characterization of Supercooled Water Droplets (–20 °C) in a Wind Tunnel

Measuring particles in subcooled environments presents specific challenges, as conventional instruments are typically designed for operation at ambient indoor temperatures. When analyzing supercooled droplets—such as those at T=–20 °C in wind tunnel conditions—optical instruments require a thermally stabilized window.

This window actively compensates for the temperature gradient between the measurement system and the cold flow field, preventing condensation, icing, and optical distortion. Additionally, it must preserve the integrity of the optical beam path, ensuring accurate measurements of particle or droplet properties under extreme thermal conditions.

The ParticleTensorAI® system is equipped with such a thermally isolated window, enabling precise, non-intrusive characterization of supercooled water droplets in harsh environments like wind tunnels.

To check whether a coating material in combination with an ultrasonic nozzle produces the target droplet size, the spray is measured by ParticleTensorAI® Even complex droplets, such as glass beads embedded in a transparent liquid, can be characterized by it.

This video demonstrates how an ultrashort femtosecond laser pulse interacts with a single droplet, revealing the temporal separation of scattering orders and their modes.

Thanks to the extreme brevity of a femtosecond pulse (1 fs = 10⁻¹⁵ s), different light scattering paths—such as reflection, refraction, and higher-order modes—can be resolved individually in time. In just 1 femtosecond, light travels only about 300 nanometers, roughly the size of a virus or 1/100th the thickness of a human hair.

This ultra-high temporal resolution is only possible because the laser pulse is so short. If the pulse were longer, all scattering orders and modes would overlap in time, making it impossible to distinguish them individually.

By using femtosecond pulses, this technique allows researchers to analyze complex scattering behaviors in droplets with unprecedented precision—offering new insights into droplet composition, structure, and dynamics.