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

TL;DR

A low-cost 3D-printed device was developed to study oil/water emulsions and how naphthenic acids affect droplet coalescence.

Contribution

A novel 3D-printed microfluidic device enables coalescence studies of oil/water emulsions without external actuation.

Findings

Naphthenic acids inhibit droplet coalescence and reduce interfacial tension.

GC×GC-HRMS revealed differences in naphthenic acid composition correlating with emulsion stability.

The 3D-printed device allows controlled droplet generation and coalescence observation in harsh conditions.

Abstract

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.

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