Extensional flow dynamics of polystyrene melt

The main objetive of this work was to experimentally study the Flow dynamics of viscoelastic fluids (Boger fluid and Hase) when they flow through a contraction/expansion system defined by a hyperbolic tube, therefore through equations analogous to the Hagen-Poiseuille equation, the pressure drop associated with the viscous interaction was quantified, and subsequently the excess pressure drop (EPD), a parameter associated with the elasticity of viscoelastic fluids, conducting comparative studies with respect to a Newtonian reference for the same shear viscosity value, which allowed observing shear speed intervals where three predominant zones were observed. The first of them of shear type coinciding with the trajectories of the Newtonian fluid of identical viscosity value, the second zone was attributed to the elastic manifestation of the fluids due to the preferential development of the extensional flow that is in constant competition with the shear flow within of the same geometry. The third zone was attributed to a predominance of the shear flow over the extensional one, because of to the fact that the hyperbolic geometry favors the development of this type of flow at high values of shear rate KEYWORDS: Excess pressure drop; Extensional flow; Hyperbolic contractions

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Effect of surfactant on motion and deformation of compound droplets in arbitrary unbounded Stokes flows

Journal of Fluid Mechanics ◽

10.1017/jfm.2016.497 ◽

2016 ◽

Vol 803 ◽

pp. 200-249 ◽

Cited By ~ 21

Author(s):

Shubhadeep Mandal ◽

Uddipta Ghosh ◽

Suman Chakraborty

Keyword(s):

Poiseuille Flow ◽

Extensional Flow ◽

Flow Dynamics ◽

Solid Particles ◽

Far Field ◽

Ambient Flow ◽

Fluid Velocities ◽

Dilute Suspension ◽

Compound Drops ◽

Migration Velocities

This study deals with the motion and deformation of a compound drop system, subject to arbitrary but Stokesian far-field flow conditions, in the presence of bulk-insoluble surfactants. We derive solutions for fluid velocities and the resulting surfactant concentrations, assuming the capillary number and surface Péclet number to be small, as compared with unity. We first focus on a concentric drop configuration and apply Lamb’s general solution, assuming the far-field flow to be arbitrary in nature. As representative case studies, we consider two cases: (i) flow dynamics in linear flows and (ii) flow dynamics in a Poiseuille flow, although for the latter case, the concentric configuration does not remain valid in general. We further look into the effective viscosity of a dilute suspension of compound drops, subject to linear ambient flow, and compare our predictions with previously reported experiments. Subsequently, the eccentric drop configuration is addressed by using a bipolar coordinate system where the far-field flow is assumed to be axisymmetric but otherwise arbitrary in nature. As a specific example for eccentric drop dynamics, we focus on Poiseuille flow and study the drop migration velocities. Our analysis shows that the presence of surfactant generally opposes the imposed flows, thereby acting like an effective augmented viscosity. Our analysis reveals that maximizing the effects of surfactant makes the drops behave like solid particles suspended in a medium. However, in uniaxial extensional flow, the presence of surfactants on the inner drop, in conjunction with the drop radius ratio, leads to a host of interesting and non-monotonic behaviours for the interface deformation. For eccentric drops, the effect of eccentricity only becomes noticeable after it surpasses a certain critical value, and becomes most prominent when the two interfaces approach each other. We further depict that surfactant and eccentricity generally tend to suppress each other’s effects on the droplet migration velocities.

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