Citation: Mustapha, K.A.; Shikh Anuar, F.; Mohd Saat, F.A.-Z. Prediction of Slip Velocity at the Interface of Open-Cell Metal Foam Using 3D Printed Foams. Colloids Interfaces 2022, 6, 80. https:// doi.org/10.3390/colloids6040080 Academic Editors: Eduardo Guzmán and Armando Maestro Received: 15 September 2022 Accepted: 1 December 2022 Published: 12 December 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/). colloids and interfaces Article Prediction of Slip Velocity at the Interface of Open-Cell Metal Foam Using 3D Printed Foams Khairul Azhar Mustapha 1 , Fadhilah Shikh Anuar 2,3, * and Fatimah Al-Zahrah Mohd Saat 1,3 1 Fakulti Kejuruteraan Mekanikal, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, Durian Tunggal, Melaka 76100, Malaysia 2 Fakulti Teknologi Kejuruteraan Mekanikal dan Pembuatan, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, Durian Tunggal, Melaka 76100, Malaysia 3 Centre of Advanced on Energy (CARe), Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, Durian Tunggal, Melaka 76100, Malaysia * Correspondence: fadhilah@utem.edu.my Abstract: An open-cell metal foam gains a lot of interest from researchers due to its unique porous structure, which provides high surface area and good tortuosity, as well as being lightweight. How- ever, the same structure also induces a massive pressure drop which requires an optimum design to suit applications, for example, a partially filled setup or staggered design. Thus, better attention to the slip velocity at the interface between the porous structure and non-porous region is required to maximize its potential, especially in thermal fluid applications. This study proposed a slip velocity model of an open-cell metal foam by using a reverse engineering method and 3D printing technology. A series of experiments and a dimensionless analysis using the Buckingham-Pi theorem were used to compute the slip velocity model. Results show that the pressure drop increases with decreasing pore size. However, the blockage ratio effects would be more significant on the pressure drop with foams of smaller pore sizes. The proposed slip velocity model for an open-cell metal foam agrees with the experimental data, where the predicted values fall within measurement uncertainty. Keywords: slip velocity; 3D printed foam; metal foam 1. Introduction An open-cell metal foam is a porous structure made of a solid matrix and intercon- nected pores. It is classified as one kind of porous media, which is promising for diverse applications such as thermal management [1], crash or sound absorption [2], and building material [3]. Initially, the open-cell metal foam can be manufactured using conventional methods such as foaming of melts by gas injection and solid-gas eutectic solidification. Later in IR 4.0, with the development of additive manufacturing technologies (3D printing), various objects could be produced faster and easier. Different kinds of material can also be used to produce the object if its STL file (3D printing format file) is made available. Hence, the unique microstructure of the open-cell metal foam can be produced using the same technology [4]. However, its performance in the thermal-fluid application should be well-understood before classifying it as a new generation of open-cell foam. Meanwhile, a closed-cell metal foam has individual enclosures within the material, which also possess ex- cellent characteristics such as optimal strength-to-weight ratio, reduced thermal expansion coefficient, and excellent energy absorption [5–8]. The closed-cell foam also exhibits lower density [9,10] and good damping properties [10]. As one would expect, the pressure drop effects can be seen as the fluid flow passes through its complicated porous structure [11–13]. Thus, a partially filled design has been considered to minimize the effects. In the proposed design, a pipe, duct, or any configuration is partly filled with a porous structure, creating two main regions: (1) free stream and (2) porous regions. A small region in between those two regions is called an interface region. Regardless of the outer geometries, the partially Colloids Interfaces 2022, 6, 80. https://doi.org/10.3390/colloids6040080 https://www.mdpi.com/journal/colloids