Effective surface dilatational viscosity of highly concentrated particle-laden interfaces

Study on the Effect of Nanoparticle Used in Nano-Fluid Flooding on Droplet-Interface Electro-Coalescence

Nano-fluid flooding is a new method capable of improving oil recovery; however, nanoparticles (NPs) significantly affect electric dehydration, which has rarely been investigated. The effect of silica (SiO₂) NPs on the droplet–interface coalescence was investigated using a high-speed digital camera under an electric field. The droplet experienced a fall, coalescence, and secondary droplet formation. The results revealed that the oil–water interfacial tension and water conductivity changed because of the SiO₂ NPs. The decrease of interfacial tension facilitated droplet deformation during the falling process. However, with the increase of particle concentration, the formed particle film inhibited the droplet deformation degree. Droplet and interface are connected by a liquid bridge during coalescence, and the NP concentration also resulted in the shape of this liquid bridge changing. The increase of NP concentration inhibited the horizontal contraction of the liquid bridge while promoting vertical collapse. As a result, it did not facilitate secondary droplet formation. Moreover, the droplet falling velocity decreased, while the rising velocity of the secondary droplet increased. Additionally, the inverse calculation of the force balance equation showed that the charge of the secondary droplet also increased. This is attributed to nanoparticle accumulation, which resulted in charge accumulation on the top of the droplet.

Dilatational viscosity of dilute particle-laden fluid interface at different contact angles

We consider a solid spherical particle adsorbed at a flat interface between two immiscible fluids and having arbitrary contact angle at the triple contact line. We derive analytically the flow field corresponding to dilatational surface flow in the case of a large ratio of dynamic shear viscosities of two fluids. Considering a dilute assembly of such particles we calculate numerically the dependence on the contact angle of the effective surface dilatational viscosity particle-laden fluid interface. The effective surface dilatational viscosity is proportional to the size and surface concentration of particles and monotonically increases with the increase in protrusion of particles into the fluid with larger shear viscosity.

Electrohydrodynamic Deformation and Rotation of a Particle-Coated Drop

A dielectric drop suspended in conducting liquid and subjected to a uniform electric field deforms into an ellipsoid whose major axis is either perpendicular or tilted (because of the Quincke rotation effect) relative to the applied field. We experimentally study the effect of surface-adsorbed colloidal particles on these classic electrohydrodynamic phenomena. We observe that at a high surface coverage (90%), the electrohydrodynamic flow is suppressed, oblate drop deformation is enhanced, and the threshold for tilt is decreased compared to that of the particle-free drop. The deformation data are explained well by a capsule model, which assumes that the particle monolayer acts as an elastic interface. The reduction of the threshold field for rotation is likely related to drop asphericity.