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Perfluoropentane droplets with cellulose nanofibers (CNF) shells have demonstrated better stability and easier surface modification as ultrasound contrast agents and drug delivery vehicles. This paper presents a theoretical model assuming a four-phase state “inverse antibubble,” with the core filled with gas perfluoropentane surrounded by liquid perfluoropentane. A continuous, incompressible, and viscoelastic stabilizing layer separates the core from the surrounding water. A parametric study is performed to predict the frequency-dependent attenuation coefficient, the speed of sound, and the resonance frequency of the droplets which have a mean diameter of 2.47 ± 0.95 μm. Results reveal that the CNF-stabilized perfluoropentane droplets can be modeled in a Rayleigh-Plesset like equation. We conclude that the shell strongly influences the acoustic behavior of the droplets and the resonance frequency largely depends on the initial gas cavity radius. More specifically, the peak attenuation coefficient and peak-to-peak speed of sound decrease with increasing shear modulus, shear viscosity, and shell thickness, while they increase with increasing gas cavity radius and concentration. The resonance frequency increases as shear modulus and shell thickness increase, while it decreases as shear viscosity and gas cavity radius increase. It is worth mentioning that droplet concentration has no effect on the resonance frequency.

We have investigated the rheology of oil-in-water emulsions stabilised in a dispersion of cellulose microfibril – biopolymer hybrid networks. In these systems, Pickering-like stabilisation from individual CMFs and bundles is provided in combination with a viscoelastic fibrillar network that prevents the droplets from coalescing. The complex modulus of emulsions in such dispersions was measured for oil concentrations ranging from 5 to 50 wt%. Models for the complex modulus of a suspension of particles in a viscoelastic matrix are used to predict the modulus of such an emulsion for different oil and microfibril concentrations. It was found that introducing the effect of the influence of the dispersed phase on the deagglomeration of the CMF after a second homogenisation step allows for a good agreement with the experimental data for both models at high concentration of cellulose microfibrils. However, at moderate oil concentrations, the model overpredicts the complex modulus, as the presence of droplet clusters decreases the value of the measured modulus.

Nano-material enhanced viscoelastic surfactant (VES) systems have attracted considerable attention as clean fracturing fluids due to their improved rheological properties compared to conventional VES systems. In this study, a pseudo-interpenetrating network VES (PINVES) fracturing fluid, which was composed of sodium oleate (NaOA), potassium chloride (KCl) and surface-modified cellulose nanofibril (SMCNF), was proposed. The rheological behavior of the PINVES fracturing fluid and the interaction mechanism between SMCNF and wormlike micelles (WLMs) were comprehensively studied. The zero-shear viscosity of PINVES increased with appropriate SMCNF addition but decreased with excess SMCNF addition. Meanwhile, the typical viscosity-flat of WLMs gradually disappeared when the concentration of SMCNF exceeded the optimum value. The optimum value was closely related to the functional groups on the surface of SMCNF. The SMCNF physically crosslinked the WLMs through hydrogen bonding but the sulfonic groups and hydrophobic groups on the surface of SMCNF destroyed the network of WLMs at relatively high SMCNF concentration. The network formed by SMCNF dominated the rheological behavior of the PINVES fracturing fluid simultaneously. In addition, due to the competition between the network of WLMs and the network of SMCNF, the PINVES fracturing fluid exhibited higher concentration dependence of surfactant in rheological behavior than conventional VES fracturing fluid. The SMCNF network also endowed the PINVES with better temperature-resistance. Furthermore, the relaxation time and elasticity of PINVES fracturing fluid increased with the increase in SMCNF concentration. The static proppant settling tests demonstrated that the addition of SMCNF improved the sand-carrying capability of PINVES fracturing fluid. The results of the study could provide some enlightenments on the application of SMCNFs in reservoir stimulation.