Schäfer, W., & Tropea, C. (2014). The time-shift technique for measurement size of non-transparent spherical particles. Proceedings of SPIE, 9232, 92320H. https://doi.org/10.1117/12.2063342
In this work, we published results in 2014 on a droplet-sizing method based on the Time-Shift Time-of-Flight (TSTOF) technique, which was developed at Technische Universität Darmstadt. The measurements focus on milk droplets generated by a rotary atomizer, a spray technology widely used in the food industry for spray drying and powder production.
What Is TSTOF?
The Time-Shift Time-of-Flight (TSTOF) technique is an optical measurement method that analyzes light-scattering signals from droplets or particles as they pass through a focused laser beam. Unlike conventional imaging or diffraction-based techniques, TSTOF does not require optical transparency or direct visual access. Instead, it evaluates the time shifts between different scattering contributions, such as reflection and refraction, to determine droplet size, velocity, and even refractive index.
The system operates in a near-backscatter configuration and requires only one-sided optical access, making it particularly well suited for industrial environments with limited measurement access.
Measurement of Suspension Droplets (e.g., Milk)
Suspension droplets such as milk present a significant challenge for traditional optical sizing methods due to their non-transparent nature and complex internal structure. Milk consists of fat globules and proteins dispersed in water, which leads to highly complex light-scattering behavior.
TSTOF is especially well suited for such conditions. By analyzing the temporal structure of the time-shifted scattering signals, the technique enables reliable extraction of droplet size and motion characteristics, even when internal scattering strongly distorts the signal. This capability is critical for quality control in spray processes, where droplet size distribution directly affects drying efficiency, product consistency, and energy consumption.