Comprehensive Evaluation of Tilted Fiber Bragg Grating (TFBG) Sensors for Characterizing Oil–Water Emulsions: A Study on the Impact of Surfactant Concentration and Mixing Speed

This article evaluates tilted fiber Bragg grating (TFBG) sensors to characterize and quantify water–oil (W/O) emulsions, a crucial aspect of the petroleum industry. The research employs a model fluid with physicochemical properties similar to crude oil, capable of producing stable W/O emulsions. The TFBG sensors were fabricated using the phase diffractive mask technique and coated with gold to analyze the impact of the surface plasmon resonance (SPR) effect. The study comprehensively evaluates TFBG sensors for characterizing and quantifying emulsions (W/O), focusing on the impact of surfactant concentration and mixing speed. The test solutions were prepared by adjusting the amounts of water, model oil, surfactant, and mixer speeds and analyzed under a microscope, revealing that as the surfactant concentration or mixing speed increases, more water droplets are dispersed in the oil, achieving greater emulsion stability. The emulsions were then introduced into a TFBG sensor, and the spectrum was collected using an OSA. The study used the envelope method, which calculates the area between the upper and lower envelopes of the spectral response, to analyze the spectral response of TFBG sensors. In the present study, TFBG sensors without gold coating exhibited no discernible variations in the envelope area across the range of water–oil emulsions investigated. Conversely, TFBG sensors with a gold overlay demonstrated a notable increase in the envelope area, contingent upon specific surfactant concentrations or mixing velocities. Curiously, the envelope area plateaued for mixing velocities equal to or exceeding 750 revolutions per minute (rpm), thereby suggesting a stabilization of the emulsion irrespective of surfactant concentration. Furthermore, a significant modulation in the envelope area was observed at surfactant concentrations of 0.1%–0.01% as the mixing velocity escalated, underscoring the pivotal role of surfactant concentration in enhancing emulsion stability. It is essential to highlight that the characteristic SPR signal was conspicuously absent upon interaction with the emulsions under study.