The use of the Janus motif in colloidal particles, i.e., anisotropic surface properties on opposite faces, has gained significant attention in the bottom-up assembly of novel functional structures, design of active nanomotors, biological sensing and imaging, and polymer blend compatibilization. This review is focused on the behavior of Janus particles in interfacial systems, such as particle-stabilized (i.e., Pickering) emulsions and foams, where stabilization is achieved through the binding of particles to fluid interfaces. In many such applications, the interface could be subjected to deformations, producing compression and shear stresses. Besides the physicochemical properties of the particle, their behavior under flow will also impact the performance of the resulting system. This review article provides a synopsis of interfacial stability and rheology in particle-laden interfaces to highlight the role of the Janus motif, and how particle anisotropy affects interfacial mechanics.
The presence of contamination in
sodium dodecyl sulfate (SDS) solutions
in the form of dodecanol (LOH) is known to drastically affect the
resulting interfacial properties such as surface tension (SFT) and
rheology. Dodecanol molecules, which are the product of SDS hydrolysis
and are inherently present in SDS solutions, have higher surface activity
compared to SDS because they are less soluble in water. A characteristic
dip in the SFT isotherm is an indicator of the dodecanol contamination
in the sample. The presence of an electrolyte in the solution impacts
the surface activity of SDS and its critical micelle concentration,
and could yield SFT isotherms that closely match those obtained for
pure SDS samples. The interpretation of the isotherms in such cases
could thus lead to misinterpretation of the surface purity. In this
work, we have examined the SFT isotherms for SDS solutions in both
the absence and presence of electrolyte. We have fitted the isotherms
to three different thermodynamic adsorption models to estimate the
amount of dodecanol present in the sample. We have applied the estimated
values for the LOH content in a two-component rheological model to
predict the viscoelasticity of such surfactant-laden surfaces. We
have compared these results with the experimentally measured interfacial
rheological properties. Our findings demonstrate that the presence
of impurities can be captured under dynamic expansion and contractions,
even for solutions containing background electrolyte.
Understanding the interactive behavior of Janus particles (JPs) is a growing field of research. The enhancement in binding energy, in comparison to homogenous particles, and the dual characteristic of JPs open up new possibilities for novel applications. In many such applications, interfacial materials become subjected to flows that produce dilational and shear stresses. Therefore, it is important to understand the impact that the Janus character brings to interfaces. In this work, we study the microstructure of two-dimensional (2D) JP monolayers formed at the air–water interface and examine the shear viscoelasticity with an interface rheometer that was adapted for in situ surface pressure control via a Langmuir trough. We extend concepts from bulk rheology to data obtained from interfacial rheology as a tool to understand and predict the monolayer’s viscoelastic behavior. Finally, by calculating the time relaxation spectrum from the measured 2D dynamic moduli, we conclude that a phenomenon similar to glass transition is taking place by analogy.
Wettability is a fundamental property that defines the fluid's distribution in oil reservoirs. Assessing wettability is required to model flow in porous media. Nevertheless, it involves complex intermolecular and surface forces. Contact angle measurement is a quantitative method to determine wettability. However, rock samples must be prepared to assure results representative of reservoir conditions. This work applies statistical analysis to investigate the relevance of variables involved in sample preparation (aging time, solvent used to remove the excess oil from the surface) and mineral type on the wettability of oil and brine from a Pre-Salt field on pure minerals. Since there is limited experimental wettability data at Pre-Salt conditions, this work aims to assist filling this gap. The results showed aging time and mineral type as the most important parameters for analysis. Furthermore, authors found that greater aging time in oil and point of zero charge of the mineral lead to a more oil-wet behavior.
Interfacial tension (IFT) between oil and brine plays a key role in determining the capillary forces in the porous medium. When studying Enhanced Oil Recovery (EOR) methods, it is of great relevance to characterize the IFT. In the case of Pre-Salt reservoirs, CO2 and water alternated with gas injections are being considered as EOR techniques. For paraffinic oils, such as alkanes, the presence of CO2 decreases the IFT between oil and brine. However, for Pre-Salt oils with high concentrations of asphaltenes and resins, the effect of CO2 injection on the oil-brine IFT has not been reported. This work uses the drop shape analysis technique to measure the IFT between a Pre-Salt crude oil and synthetic brine with the composition of formation water in the presence and absence of CO2. The results were compared to those obtained for synthetic oil consisting of alkane and aromatic molecules. For the crude oil, CO2 dissolution, which decreases brine pH, increased the IFT between oil and brine. Oil characterization retrieved high concentration of asphaltenes and resins and considerable acid and basic numbers. In addition, infrared spectroscopy and nuclear magnetic resonance of the asphaltene fractions of the crude oil reported acid functional groups in these polar compounds. Therefore, the surface activity of the polar compounds in the oil may be reduced at lower pH. On the other hand, for the synthetic oil, CO2 decreased the IFT as previously reported for alkane molecules. Therefore, this work shows the difference in the effect of CO2 on IFT, which depends on the composition of the oil and aqueous phases. Furthermore, the acid/base characterization of the polar compounds is relevant to understand the effect of CO2 dissolution on the resulting IFT.
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