Comparison between shadow imaging and in-line holography for measuring droplet size distributions

Abstract

A direct comparison of the droplet size and number measurements using in-line holography and shadow imaging is presented in three dynamically evolving laboratory scale experiments. The two experimental techniques and image processing algorithms used to measure droplet number and radii are described in detail. Droplet radii as low as $$r = 14$$ r = 14 µm are measured using in-line holography and $$r = 50$$ r = 50 µm using shadow imaging. The droplet radius measurement error is estimated using a calibration target (reticle) and it was found that the holographic technique is able to measure droplet radii more accurately than shadow imaging for droplets with $$r \le 625$$ r ≤ 625 µm. Using the measurements of droplet number and size we quantitatively cross-validate and assess the accuracy of the two measurement techniques. The droplet size distributions, N(r), are measured in all three experiments and are found to agree well between the two measurement techniques. In one of the laboratory experiments, simultaneous measurements of droplets ($$r \ge 14$$ r ≥ 14 µm, using holography) and dry aerosols ($$0.07 \lessapprox r \lessapprox 2$$ 0.07 ⪅ r ⪅ 2 µm, using an scanning mobility particle sizer and $$0.15 \lessapprox r \lessapprox 5$$ 0.15 ⪅ r ⪅ 5 µm using an optical particle sizer) are reported, one of the first such comparison to the best of our knowledge. The total number and volume of droplets is found to agree well between both techniques in the three experiments. We demonstrate that a relatively simple shadow imaging technique can be just as reliable when compared to a more sophisticated holographic measurement technique over their common droplet radius measurement range. The agreement in results is shown to be valid over a large range of droplet concentrations, which include experiments with relatively sparse droplet concentrations as low as 0.02 droplets per image. Advantages and disadvantages for the two techniques are discussed in the context of our results. The main advantages to in-line holography are the greater accuracy in droplet radius measurement, greater spatial resolution, larger depth of field, and the high repetition rate and short pulse duration of the laser light source. In comparison, the main advantages to shadow imaging are the simpler experimental setup, image processing algorithm, and fewer computer resources necessary for image processing. Droplet statistics like number and size are found to be very reliable between the two methods for large range of droplet densities, $${\mathcal {P}}_{r>50}$$ P r > 50 , ranging from $$10^{-4} \le {\mathcal {P}}_{r>50} \le 10^{-1}$$ 10 - 4 ≤ P r > 50 ≤ 10 - 1 cm$$^{-3}$$ - 3 , when the two techniques are implemented as shown in this paper.

Method for Measuring the Three-Dimensional Morphology of Near-Wall Bubbles and Droplets Based on LED Digital Holography

“…During droplet evaporation, the morphology changes, encompassing alterations in physical and chemical properties, such as surface tension, contact angle, and contact radius. Due to the short coherence length of LED, current holographic measurement techniques for droplet morphology often employ the method of coaxial phase-shifting recording. , However, coaxial systems can solely measure stationary objects or slowly change dynamic processes. In this regard, the off-axis holography developed in this study is better suited for capturing the rapid changes in the dynamic evaporation process of droplets.…”

Plunging breakers. Part 2. Droplet generation

“…The curves are a bit noisy but generally indicate an increasing droplet number with increasing breaker strength at diameters above approximately 400 µm, the curves converge at smaller diameters. A number of authors have measured droplet diameter distributions in breaking wave and wind wave systems in the laboratory (see for example Veron et al 2012;Ortiz-Suslow et al 2016;Erinin et al 2022;Ramirez de la Torre et al 2022), the field (Wu et al 1984;Monahan 1968) and in DNSs with and without wind (Wang et al 2016;Tang et al 2017;Mostert et al 2022). Many of the distributions in these studies are either fitted or compared with separate power-law functions at small and large droplet diameter ranges.…”

“…Veron et al (2012) studied spray generation at high wind speeds (31.3-47.1 m s −1 ) and found that the numbers of large droplets exceeded theoretical predictions. More recently, Erinin et al (2022) measured droplet speed and acceleration statistics of spray generated at wind speeds up to 12 m s −1 in a wind wave field in a laboratory tank and reported on droplet speed and acceleration probability density functions. The authors found droplets with speeds greater than the measured wind speed.…”