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.

ParticleTensorAI® – AI-based Particle and Droplet Analysis

ParticleTensorAI® is an AI-driven analysis platform for the characterization of particles and droplets based on optical measurement data. The system converts continuous light-scattering or imaging signals into tensor-based representations, which are evaluated using convolutional neural networks (CNNs) and statistical methods. This enables calibration-free, real-time determination of particle properties such as size, concentration, and distribution, even in complex environments like sprays and dense suspensions.

In this video, a 5 × 3 matrix of ParticleTensorAI (PTA) outputs from different experiments is shown, illustrating the robustness and consistency of the tensor-based representation across varying experimental conditions. The visualization highlights how ParticleTensorAI® captures characteristic signal patterns that can be directly analyzed by AI without preprocessing.

Here is a video related to our publication [1], where we discuss the laser-based measurement technique TSTOF for particle and droplet characterization and compare it with established methods such as LDT [2] and PDA [3,4].

In my view, the key distinction is that TSTOF is based on the analysis of incoherent light scattering from individual droplets. This enables characterization not only of droplet size and velocity, but also of droplet composition.

In the video, you can see a real-time measurement of paint droplets using a TSTOF-based device. Paint droplets typically consist of multiple materials, which makes their characterization—especially in terms of composition—particularly challenging.

References

[1] Schaefer, W.; Li, L.; Stegmann, P.; Terada, M. Technical Report on the TSTOF Measurement Method: Technical Basics, Historical Development, and Comparison with Other Laser-Based Measurement Methods. Photonics 2026, 13, 56. https://doi.org/10.3390/photonics12070673

[2] Swithenbank, J.; Beer, J.M.; Taylor, D.S.; Abbot, D.; McCreath, G.C.A Laser Diagnostic Technique for the Measurement of Droplet and Particle Size Distribution. In Proceedings of the 14th AIAA Aerospace Sciences Meeting, Washington, DC, USA, 26–28 January 1976; AIAA Paper 76-79.

[3] Bachalo, W.D. Method for measuring the size and velocity of spheres by dual-beam light-scatter interferometry. Appl. Opt. 1980, 19, 363.

[4] Flögel, H.H. Modifizierte Laser-Doppler-Anemometrie zur Simultanen Bestimmung von Geschwindigkeit und Größe Einzelner Partikeln; VDI Verlag: Düsseldorf, Germany, 1987.

Conference contributions 19.31 [DECHEMA 2026, Karlsruhe, Germany] #milkspray #spray #tstof
https://youtube.com/shorts/4Jgjfoeetvg?feature=share

In this video, we demonstrate how to measure the spray angle of a flat-fan nozzle. For this purpose, a measurement head is moved periodically back and forth across the spray.

Integrated position sensors continuously record the measurement position together with data from the spray measurement device, including droplet size, velocity, and droplet count. In addition, the nozzle flow rate and pressure are recorded. Based on these measurements, the local spray density is calculated, allowing the spray angle to be determined dynamically.

By varying the pump flow rate, the spray angle can be evaluated at different operating pressures. In this experiment, we use the SprayQuantAI® probe for spray characterization and a Lechler flat-fan nozzle to generate the spray.

Research 29.29 [Light Scattering from LN1703 Setup]

https://youtube.com/shorts/o8aCXeWZ6Qc

In this video, we show how the spray cone of a pneumatic atomizer can be experimentally characterized using SprayConeAI®.

SprayConeAI® is a measurement method developed to analyze the geometrical distribution of droplets within a spray cone. The scientific foundation of this approach was first published in 2016 [1], where the concept of the spray matrix was introduced.

The spray matrix is generated using a TSTOF measurement device [2]. It provides spatially resolved information on droplet size, droplet velocity, and droplet number density throughout the spray. By continuously traversing the spray nozzle relative to the measurement volume, the complete spray cone can be systematically mapped. This enables a detailed understanding of how droplet properties evolve within the spray field.

SprayConeAI® also offers optional tools for analyzing scanned transparent foils. These tools allow the calculation of coating thickness distribution and spray cone geometry, creating a direct link between measured droplet dynamics and the resulting coating quality.

SprayConeAI® is continuously evolving. Follow our Telegram channel to stay up to date.

[1] Schäfer, W., Rosenkranz, S., Brinckmann, F., & Tropea, C. (2016). Analysis of pneumatic atomizer spray profiles. Particuology, 29, 80–85. https://doi.org/10.1016/j.partic.2015.12.002

[2] Schaefer, W., Li, L., Stegmann, P., & Terada, M. (2026). Technical report on the TSTOF measurement method: Technical basics, historical development, and comparison with other laser-based measurement methods. Photonics, 13(1), 56. https://doi.org/10.3390/photonics13010056

CERN 2026