A Hydrophilic 3D-Printed Microfluidic Device for Emulsion Studies: Preliminary Observations on the Role of Naphthenic Acids in Coalescence

Produced water (PW) comes from oil production and contains both nonpolar and polar substances that can be dissolved or dispersed in the aqueous phase. Such a complex mixture is typically found as an oil/water emulsion (O/W), which can be stabilized by the presence of interfacial materials, such as naphthenic acids (NAs). Recent literature has focused on isolating and characterizing the acidic interfacial material, but our fundamental understanding of the role of NAs in emulsion stabilization and coalescence remains scarce. This is justified by the challenges associated with droplet formation in microfluidics. For instance, the high viscosity of the O/W affects the dominant forces in the pinch-off, as well as wetting conditions due to surface chemistry compatibility with O/W, thereby limiting the efficiency of droplet generation. In addition, most reported devices rely on external actuation to induce coalescence, with few design options currently available. In this context, enabling technologies are desperately needed to probe the coalescence of complex O/W systems and to allow for the correlation of compositional data at the molecular level with the stability of O/W. In this study, a 3D-printed microfluidic device was created using an LCD-based SLA printer and a custom hydrophilic and transparent resin suitable for coalescence studies with O/W. The device features channels of ∼40 μm for consistent droplet generation and coalescence observation without requiring surface modifications or external actuation to induce coalescence. The device was tested using a model O/W system, demonstrating controlled droplet generation and observation of coalescence phenomena. The effect of NAs on emulsion stability was investigated by analyzing mixtures of NAs from both crude oil and produced water. It was found that NAs inhibited droplet coalescence, which was consistent with their role as surfactants, and reduced interfacial tension. Comprehensive two-dimensional gas chromatography coupled with high-resolution mass spectrometry (GC×GC-HRMS) was used to characterize the molecular composition of NAs in the samples for further interpretation of the coalescence data. The GC×GC-HRMS results revealed differences in carbon number distribution and degree of cyclization of NAs, allowing for correlation with observed variations in emulsion stability. This study showed that 3D-printed devices can be engineered for droplet generation and coalescence investigations using a single, disposable device. This report is particularly important in showcasing the new and upcoming applications of inexpensive (less than 1 US dollar) polymeric 3D-printed devices under harsh environments that would otherwise cause substrate swelling.