Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userStretching of viscoelastic drops by steady sliding
86
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
The sliding of non-Newtonian drops down planar surfaces results in a complex, entangled balance between interfacial forces and non linear viscous dissipation, which has been scarcely inspected. In particular, a detailed understanding of the role played by the polymer flexibility and the resulting elasticity of the polymer solution is still lacking. To this aim, we have considered polyacrylamide (PAA) solutions of different molecular weights, suspended either in water or glycerol/water mixtures. In contrast to drops with stiff polymers, drops with flexible polymers exhibit a remarkable elongation in steady sliding. This difference is most likely attributed to different viscous bending as a consequence of different shear thinning. Moreover, an optimal elasticity of the polymer seems to be required for this drop elongation to be visible. We have complemented experimental results with numerical simulations of a viscoelastic FENE-P drop. This has been a decisive step to unravel how a change of the elastic parameters (e.g. polymer relaxation time, maximum extensibility) affects the dimensionless sliding velocity.
rate research
Read More
We study numerically the deformation of sessile dielectric drops immersed in a second fluid when submitted to the optical radiation pressure of a continuous Gaussian laser wave. Both drop stretching and drop squeezing are investigated at steady state where capillary effects balance the optical radiation pressure. A boundary integral method is implemented to solve the axisymmetric Stokes flow in the two fluids. In the stretching case, we find that the drop shape goes from prolate to near-conical for increasing optical radiation pressure whatever the drop to beam radius ratio and the refractive index contrast between the two fluids. The semi-angle of the cone at equilibrium decreases with the drop to beam radius ratio and is weakly influenced by the index contrast. Above a threshold value of the radiation pressure, these optical cones become unstable and a disruption is observed. Conversely, when optically squeezed, the drop shifts from an oblate to a concave shape leading to the formation of a stable optical torus. These findings extend the electrohydrodynamics approach of drop deformation to the much less investigated optical domain and reveal the openings offered by laser waves to actively manipulate droplets at the micrometer scale.
We present a tube model for the Brownian dynamics of associating polymers in extensional flow. In linear response, the model confirms the analytical predictions for the sticky diffusivity by Leibler- Rubinstein-Colby theory. Although a single-mode DEMG approximation accurately describes the transient stretching of the polymers above a sticky Weissenberg number (product of the strain rate with the sticky-Rouse time), the pre-averaged model fails to capture a remarkable development of a power-law distribution of stretch in steady-state extensional flow: while the mean stretch is finite, the fluctuations in stretch may diverge. We present an analytical model that shows how strong stochastic forcing drive the long tail of the distribution, gives rise to rare events of reaching a threshold stretch and constitutes a framework within which nucleation rates of flow-induced crystallization may understood in systems of associating polymers under flow. The model also exemplifies a wide class of driven systems possessing strong, and scaling, fluctuations.
Variable power transmission in mechanical systems is often achieved by devices, e.g., clutches and brakes, that use dry friction. In these systems, the variability in power transmission is brought about by engaging and disengaging the friction plates. Though commonly used, this method of making the coupling noisy is not as versatile as their electrical analog. An alternative method would be to intermittently vary the frictional force. In this paper, we demonstrate a self-organized way to tune the noise in the frictional coupling between two surfaces which are in relative motion with each other. This is achieved by exploiting the complexity that arises from the frictional interaction of the balls which are placed in a circular groove between the surfaces. The extent of floppiness in the coupling is related to the rate at which the balls make transitions between their rolling and sliding states. If the moving surface is soft and the static surface is hard we show that with increasing filling fraction of the balls the transitions between rolling and sliding against the static surface give way to the transitions between rolling and sliding against the moving surface. As a consequence, the noise in the coupling is large for both small and large filling fraction with a dip in the middle. In contrast, the sliding with the static surface is suppressed if the moving pate is hard and the noise in the coupling decreases monotonically with the filling fraction of the balls.
In this paper we present experimental and numerical studies of the electrohydrodynamic stretching of a sub-millimetre-sized salt water drop, immersed in oil with added non-ionic surfactant, and subjected to a suddenly applied electric field of magnitude approaching 1 kV/mm. By varying the drop size, electric field strength and surfactant concentration we cover the whole range of electric capillary numbers ($Ca_E$) from 0 up to the limit of drop disintegration. The results are compared with the analytical result by Taylor (1964) which predicts the asymptotic deformation as a function of $Ca_E$. We find that the addition of surfactant damps the transient oscillations and that the drops may be stretched slightly beyond the stability limit found by Taylor. We proceed to study the damping of the oscillations, and show that increasing the surfactant concentration has a dual effect of first increasing the damping at low concentrations, and then increasing the asymptotic deformation at higher concentrations. We explain this by comparing the Marangoni forces and the interfacial tension as the drops deform. Finally, we have observed in the experiments a significant hysteresis effect when drops in oil with large concentration of surfactant are subjected to repeated deformations with increasing electric field strengths. This effect is not attributable to the flow nor the interfacial surfactant transport.
We report on a new mode of self-propulsion exhibited by compact drops of active liquids on a substrate which, remarkably, is tractionless, i.e., which imparts no mechanical stress locally on the surface. We show, both analytically and by numerical simulation, that the equations of motion for an active nematic drop possess a simple self-propelling solution, with no traction on the solid surface and in which the direction of motion is controlled by the winding of the nematic director field across the drop height. The physics underlying this mode of motion has the same origins as that giving rise to the zero viscosity observed in bacterial suspensions. This topologically protected tractionless self-propusion provides a robust physical mechanism for efficient cell migration in crowded environments like tissues.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
