The response of polyelectrolytes (PEs) to applied electric fields drives applications in energetics, diagnostics, materials development, and many more. Here we employ allatom molecular dynamics (MD) simulations to probe the response of grafted PE brushes to axial electric fields. For PEs with large charge densities, the electric field triggers a left−right asymmetric distribution of counterions around the PE backbone: consequently, depending on the location (left or right), there is an unequal screening and an unequal force on the PE functional groups causing a bending-driven brush height reduction. However, for the weakly charged PEs, the electric field causes a uniform distribution of the counterions across the brush, enforcing a uniform (without left−right asymmetry) partially unscreened PE brush: therefore, all the brush segments experience similar force causing a brush tilting-driven brush height reduction. Such bending versus tilting responses is commensurate with the electric-field-driven increase (decrease) in the flexibility of strongly (weakly) charged PEs.
The origins of free surface vortex
and gas entrainment are investigated
within a liquid pool using a rotating cylindrical disc having longitudinal
axis normal to the gas–liquid interface with varying submergence
from 1.40 to 2.85 times the disc radius (20 mm). A generalized vortex
profile is obtained by suitable scaling for a range of rotational
Froude numbers (2.38–11.18). The transient evolution rate of
the vortex tip followed logarithmic law and is correlated with experimental
data of air and glycerin. The axial pull that holds the vortex against
gravity is found to be linearly increasing with the rise of disc rotation.
Asymmetric vortex profiles with increased extent of asymmetry are
obtained upon increase of disc inclination (0–17.45°).
Axial and radial circulations are revealed by following the trajectory
of a solid particle in the flow field. Entrainment of discrete air
volumes is observed from the vortex core at the corresponding Froude
number and submergence ratio.
In this paper, we develop one of the first models for closed-form fully analytical solutions for describing the nonionic and ionic diffusio-osmotic (DOS) transport at interfaces grafted with a soft and porous polymeric film in the presence of a neutral solute concentration gradient (for nonionic diffusio-osmosis) and a salt concentration gradient (for ionic diffusio-osmosis). The nonionic DOS velocity depends on this solute concentration gradient and the drag force from the polymeric film. The ionic DOS transport is characterized by the diffusio-osmotically induced electric field and the diffusio-osmotically induced velocity field. This induced electric field is primarily dictated by the conduction of the mobile ion imbalance present within the electric double layer, induced at the charged solid, in the presence of the applied salt concentration gradient. The DOS velocity, on the other hand, is driven by a combination of the induced pressure gradient and an induced electro-osmotic body force (triggered by this induced electric field) and is opposed by the drag from the polymer layer. The result is a velocity field whose magnitude increases rapidly at near wall locations, decreases away from the wall, and depending on the salt concentration, may or may not increase outside the polymeric layer. This unique velocity profile ensures the presence of significant hydrodynamic shear stress across a wide zone extending from the wall in a non-confined fluidic system: This will ensure that finite-sized species (e.g., biological cells) can be conveniently made to access locations of large hydrodynamic stresses for a myriad of engineering and biological applications.
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