Vol. 54 No. 9 2021 517 Copyright © 2021 The Society of Chemical Engineers, Japan Journal of Chemical Engineering of Japan, Vol. 54, No. 9, pp. 517–524, 2021 Preparation of Polystyrene Microcapsules Containing Saline Water Droplets via Solvent Evaporation Method and Their Structural Distribution Analysis by Machine Learning Sukhbaatar Batchuluun 1 , Hideki Matsune 2 , Koichiro Shiomori 2 , Ochirkhuyag Bayanjargal 3 and Tserenkhand Baasankhuu 3 1 Interdisciplinary Graduate School of Agriculture and Engineering, University of Miyazaki, 1-1 Gakuenkibanadai-nishi, Miyazaki-shi, MIyazaki 889-2192, Japan 2 Department of Applied Chemistry, University of Miyazaki, 1-1 Gakuenkibanadai-nishi, Miyazaki-shi, MIyazaki 889-2192, Japan 3 Department of Chemical and Biological Engineering, School of Engineering and Applied Sciences, National University of Mongolia University, Street-1, Ulaanbaatar-14201, Mongolia Keywords: Microcapsules, Solvent Evaporation, S/O/W Emulsion, Structural Distribution, Machine Learning Image Analysis Most microcapsule preparation methods produce a population of microcapsules in a bulk solution. To control the micro- capsule preparation or obtain an optimal preparation condition, the mechanism of the microcapsule preparation should be investigated. The mechanism is estimated via structure reformation during the preparation process because diameter and wall thickness are drastically altered in the solution. Considering microcapsule applications, some important proper- ties, such as the mechanical properties of microcapsules and release rate of the encapsulated product, depend on the microcapsule structure. In this study, polystyrene microcapsules containing saline water droplets were prepared via the solvent evaporation method from a solid-in-oil-in-water (S/O/W) emulsion system. The microcapsules exhibited a specific structural distribution, which comprised monocore, multicore, and solidcore structures. The structural distribu- tion was altered by the preparation condition. The monocore structure was absolutely dominant owing to the increase in the amount of calcium chloride added in the organic phase. The salt concentration is not the sole controlling factor of the microcapsule structure, as the surfactant and dispersion exerted a significant impact on the microcapsule structure. The structural distribution was automatically analyzed by a machine learning algorithm (MLA). The decision-making time for the microcapsules preparation was shortened by the accelerated structure determination, and the accuracy was improved by increasing the number of counting particles. Introduction A complex emulsion system such as a water-in-oil-in- water (W/O/W) emulsion is widely adopted in a solvent evaporation method for microencapsulation (Ijichi et al., 1997; Kiyoyama et al., 2003). e complex emulsion system exhibits an osmotic behavior because the system is a liq- uid membrane, owing to the continuous solvent molecule evaporation and the mutual solubility of the system com- ponents. e osmotic behavior of the complex system plays an important role in the structure of the microcapsule. In addition, an aqueous solution is widely used in various in- dustries, such as food, cosmetics, dyes, and pharmaceutical industries. erefore, a water containing microcapsule was prepared by researchers (Kentepozidou and Kiparissides, 1995; Taguchi et al., 2019), using the W/O/W system. How- ever, the S/O/W emulsion system is not usually adopted, except in few studies, including our previous study, which focused on the preparation and release characteristics of biodegradable microcapsules encapsulating activated carbon impregnated with pesticide using the solvent evaporation method (Shiomori et al., 2004). A multiple emulsion system is the result of double dispersion. Specifically, in this partic- ular system, which is the S/O/W system, the S/O suspension is initially formed during the solid particle distribution in the oil phase, and then, the resulting emulsion is dispersed in the aqueous media. Here, during the second step, the S/O/W system changed to the W/O/W system because of the osmotic flow of water. Hence, two surfactants are gener- ally required in the W/O/W system. In our previous study (Kiyoyama et al., 2003; Shiomori et al., 2004), multicore mi- crocapsules were prepared by the complex emulsion system stabilized with a cosurfactant and via the Pickering method. It is necessary to control the internal structure, especially for a given durability. For example, a microcapsule inside concrete must be broken at the time a concrete microcrack is formed, and an active agent is released from inside the microcapsule for self-healing. However the microcapsule must not be ruptured during the mixing of the concrete raw material. erefore, the structure of microcapsules plays a crucial role in releasing the profile, as well as in the strength Received on May 24, 2021; accepted on June 30, 2021 DOI: 10.1252/jcej.21we052 Correspondence concerning this article should be addressed to K. Shio- mori (E-mail address: shiomori@cc.miyazaki-u.ac.jp). Research Paper