Probing Interfacial Water-in-Crude Oil Emulsion Stability Controls Using Electrorheology

Abstract

The stability of water-in-oil emulsions is controlled by several interfacial mechanisms that include the oil film rheology between approaching drops and the resistance to rupture of drop interfaces or a combination of these two interfacial controls. Film drainage is mainly a function of the continuous phase rheology. Temperature is regulated to control the viscosity of the continuous phase and, hence, determine its effect on emulsion stability through film drainage, in contrast with interfacial resistance to rupture. In this study, one crude oil is used to formulate water-in-oil emulsions. Oil-water interfacial tensions are measured to gauge other interfacial changes with temperature. The influence of cation type in the aqueous phase, which have been determined to affect emulsion stability, is examined by comparing the stability of emulsions made with either purely monovalent or divalent aqueous solutions with the same overall ionic strength. Divalent cations in water contribute to form stronger emulsions than those produced with purely monovalent aqueous phase. The critical field value, used as proxy of emulsion stability, approaches a plateau value for NaCl emulsions and slowly decreases for CaCl 2 , at sufficiently high temperature (50 °C), which is interpreted here to reflect the intrinsic drop-coating film resistance to coalescence. Interfacial tension does not vary significantly with either aqueous phase composition or temperature. From the comparison with previous results using a less asphaltic oil, it is speculated that the drop coating film is composed of a fraction of asphaltenes, for the crude oil studied here.

On the flow topology inside droplets moving in rectangular microchannels

“…34 On the contrary, plugs (a > b = w) experience increased contact with the 20 channel walls, allowing the continuous phase to bypass them only through the corners; a long thin film layer is formed between the channel walls and the droplet exerting substantial shear forces (drag) on the droplet and causing its velocity v d to reduce compared to that of spherical droplets and to become closer to v c . 25 The mobility of pancake droplets (d < a = b < w) lies between those of spherical droplets and plugs due to contact with two of the channel walls. Within the studied Ca range, the velocity ratio is always greater than unity, but the rate of increase in mobility with increasing Ca in plugs is smaller compared to spherical 30 droplets and pancakes.…”

“…For each case, 100 image pairs were acquired, from which 30 -50 image pairs were selected and preprocessed using the above algorithm. The coefficient of variation 25 of the droplet velocities was 2% ± 4%. The time between image pairs was adjusted for each flow rate.…”

“…Deionised water was used in all aqueous phases. Microfluidic polydimethylsiloxane (PDMS) devices were fabricated by 25 combined photo-and soft lithography. 21 Detailed channel architectures were designed in AutoCAD (AutoDesk, USA) and standard photolithography techniques were used for fabrication as described by Bauer et al 22 The channel dimensions are shown in Fig.…”

“…It should be noted that according to Guillot et al, 35 the interface shear viscosity term can be neglected when considering the shear stresses at the liquid-liquid interface, as the interface shear 25 viscosity tends to be substantially smaller than the fluid viscosity in the presence of surfactant monolayers at the w/o interface. It is clear that the boundary conditions dictate the droplet flow topology to a large extent and the viscosity ratio λ = (∂u/∂y) c /(∂u/∂y) d plays a key role.…”

“…Droplet-based microfluidics has evolved as an important platform in many aspects of scientific research, due to its ability to isolate targets from their surroundings through (most commonly) w/o 25 interfaces. Understanding of the flow pattern inside the droplets is required for successful application of the technology in, for example, chemical synthesis [1][2][3][4] and single cell screening applications.…”