Experimental Methods for Measuring the Breakup Frequency in Turbulent Emulsification: A Critical Review

Abstract

The growing interest in using population balance modeling to describe emulsification processes has spurred an interest in experimentally measuring the breakup frequency. This contribution classifies, compares, and critically reviews the different methods that have been suggested for measuring the breakup frequency, applicable to emulsification devices. Two major approaches can be seen in previous studies. The first is ‘single drop breakup experiment’-based studies, which estimate the breakup frequency by observing the fate of individual drops. The second approach involves ‘emulsification experiment’-based studies, which combine measured drop-size distributions with assumptions to allow for estimations of the breakup frequency. This second approach can be further subdivided in three types: Parametric determination, inverse self-similarity-based methods, and direct back-calculation methods. Each of these methods are reviewed in terms of their implementation, reliability, and validity. Suggestions of methodological considerations for future studies are given for each class, together with more general suggestions for further investigations. The overall objective is to provide emulsification researchers with background information when choosing which method to use for measuring the breakup frequency and with support when setting up experiments and data evaluation procedures.

Estimating Breakup Frequencies in Industrial Emulsification Devices: The Challenge of Inferring Local Frequencies from Global Methods

“…Several methods have been suggested for estimating the global breakup rate from emulsification experiment data (a review is available elsewhere [24]). The conceptually simplest one is to estimate the breakup frequency of the largest size class containing drops using a direct back-calculation [15,47].…”

“…Using Equation ( 8) to estimate g* will be referred to as the 'uncorrected global estimation method' in this study (Note that the more complex back-calculation method proposed by Vankova et al [30] is a generalization of this approach [24]).…”

“…Consequently, there is a need for measuring breakup frequencies, especially in industrially relevant emulsification devices. A large number of experimental method for measuring breakup frequency have been suggested (see recent review [24]). These methods can be divided in two broad classes: The first class is the "single drop breakup experiments" where an experimental model device is designed to allow optical access with high-speed photography of breaking drops.…”

“…These difficulties have long been realized, and several methods have been suggested for estimating breakup frequencies directly from the drop-size distributions resulting from emulsification processes. This class of breakup frequency estimation methods includes three sub-classes [24]; (i) parameter fitting to PBE [9,25,26], (ii) inverse self-similarity based methods [27][28][29] and (iii) direct back-calculation methods [15,30,31].…”

“…However, such an approach has several disadvantages. Since there is yet no consensus on best practices for these methods [24], it would be difficult to separate systematic differences from experimental uncertainty. Moreover, it would be difficult to conclude on what causes the systematic differences between local and global estimates that would appear in such a comparison, if using a purely experimental approach.…”