  Citation: Chiriac, E.; Avram, M.; Balan, C. Investigation of Multiphase Flow in a Trifurcation Microchannel—A Benchmark Problem. Micromachines 2022, 13, 974. https://doi.org/10.3390/ mi13060974 Academic Editors: Sangjin Ryu, Moeto Nagai and Seunghee Kim Received: 20 May 2022 Accepted: 18 June 2022 Published: 20 June 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). micromachines Article Investigation of Multiphase Flow in a Trifurcation Microchannel—A Benchmark Problem Eugen Chiriac 1,2, * , Marioara Avram 1 and Corneliu Balan 2 1 Laboratory for Micro- and Nano- Fluidics- L10, National Institute for R&D in Microtechnologies—IMT Bucharest, 126A, Erou Iancu Nicolae Street, Ilfov, 077190 Voluntari, Romania; marioara.avram@imt.ro 2 REOROM Laboratory, Faculty of Power Engineering, University “Politehnica” of Bucharest, 313, Splaiul Independent , ei, Sector 6, 060042 Bucharest, Romania; corneliu.balan@upb.ro * Correspondence: eugen.chiriac@imt.ro Abstract: The evolution of an interface between two immiscible liquids in a three-branch symmetric microchannel is numerically and experimentally investigated. The main goals of the paper are to correlate the numeric data with the experimental results for the tested flow case and to assess the quality of the VOF procedure to trace the interface using the Fluent commercial code. The focus of the experiments was to characterize the dynamics of the oil–water interface formed in the vicinity of the bifurcation, at the entrance in the main microchannel of 400 microns width and 50 microns height. The oil core surrounded by water is visualized and micro-PIV measurements are performed in water. Experimental results qualitatively and quantitatively confirm the 3D numerical simulations. We propose the present investigated flow as a benchmark case for the study of the interface in a branching microchannel geometry. Keywords: microfluidics; CFD; multiphase flow; interface 1. Introduction In multiphase flow, the validation and verification of the flow field, and the presence of multiple interfaces in the microchannel are topics of high importance in computational fluid dynamics. The applications of multiphase microfluidics are found in domains such as chemical synthesis [1], bioanalysis [2], 3D printing [3], and graphene field-effect transistors (GFET) integrated into microchannels [4] to create sensors with high sensitivity [5]. The ad- vantage of linking immiscible fluids with microfluidics leads to important advancements in the domain of a 3D cell culture [6], where spheroids can be quantified and analyzed by a data-driven approach [7]. In the domain of chemical engineering, the separation of phases [8] plays an important role in the extraction of ions [9,10]. When the fluids are immiscible, there are several methods used to model the interface. Two main directions can be followed: surface methods and volume methods [11]. In general, in both methods, the one-fluid formulation is used for the Navier–Stokes equation where a source term is added to take into account the effects of the surface tension, along with a differential equation that has to be solved at the interface. For the surface methods or interface tracking methods, the interface is either marked with particles or represented by the grid, and it is advected by the fluid at the interface. For the volume methods, or interface capturing methods, the interface is reconstructed by solving a transport equation at the interface. There are certain volume methods: markers in fluid, volume of fluid, level-set method [12], and hybrid methods [13]. Another technique that can be used to solve the multiphase flow is the lattice Boltzmann color gradient method [14], where the macroscopic flow system is predicted by simulating the discrete fluid elements. Despite its applicability, the main drawback of the method is that numerical errors may arise when there is a high contrast between the material properties of the fluids used. In this work, we focused on the volume of fluid (VOF) method and its mechanisms are presented in [15]. Micromachines 2022, 13, 974. https://doi.org/10.3390/mi13060974 https://www.mdpi.com/journal/micromachines