Laboratory-based instruction is a powerful educational tool that engages students in Science, Technology, Engineering and Mathematics (STEM) disciplines beyond textbook theory. This is true in mechanical engineering education and is often used to provide collegiate-level students a hands-on alternative to course theory. Module-based laboratory instruction allows students to investigate fundamental concepts interactively and often affords new critical thinking skills and technical aptitude. The authors have developed a novel mass conservation laboratory module for use in undergraduate fluid mechanics education via module-based instruction. The module investigates mass conservation fundamentals in a simple microfluidic T-junction device. The experiment is a novel application of microfluidics-based instruction, is highly repeatable, and can be conducted at relatively low cost. The module exposes students to the rapidly developing field of microfluidics and allows them to gain familiarity with fluorescence-based optical diagnostics and simple signal processing. In addition, this study quantifies the module's educational impact on thirty-six mechanical engineering undergraduates. A baseline study was conducted by utilizing knowledge assessments before and after the experimental module. The results of the study are statistically significant and suggest the module's efficacy for teaching mass conservation fundamentals in an undergraduate curriculum.