Witten Thomas A, University of Chicago , USA.
Diamant Haim A, Tel Aviv University , Israel.,
20132016,
USIsrael Binational Science Foundation,.
powerpoint 9 megabytes
Screening, hyperuniformity, and instability in the sedimentation of irregular objects
Accepted in Phys. Rev. Lett 23Mar 2017 ,
Abstract:We study the overdamped sedimentation of nonBrownian objects of irregular shape using fluctuating hydrodynamics. The anisotropic response of the objects to flow, caused by their tendency to align with gravity, directly suppresses concentration and velocity fluctuations. This allows the suspension to avoid the anomalous fluctuations predicted for suspensions of symmetric spheroids. The suppression of concentration fluctuations leads to a correlated, hyperuniform structure. For certain object shapes, the anisotropic response may act in the opposite direction, destabilizing uniform sedimentation. Swirling a liquid from within


Hydrodynamic Interactions between Two Forced Objects of Arbitrary Shape: II Relative Translation
(Submitted to Phys. Rev. E on 30 Nov 2015) ,
Abstract: We study the relative translation of two arbitrarily shaped objects, caused by their hydrodynamic interaction as they are forced through a viscous fluid in the limit of zero Reynolds number. It is well known that in the case of two rigid spheres in an unbounded fluid, the hydrodynamic interaction does not produce relative translation. More generally such an effective pairinteraction vanishes in configurations with spatial inversion symmetry, for example, an enantiomeric pair of arbitrary shape does not develop relative translation instantaneously. We show that the breaking of inversion symmetry by boundaries of the system accounts for the interactions between two spheres in confined geometries, as observed in experiments. The same general principle also provides new predictions for interactions in other object configurations near obstacles. We examine the timedependent relative translation of two selfaligning objects, extending the numerical analysis of our preceding publication [Goldfriend, Diamant and Witten, arXiv:1502.00221 (2015)]. The interplay between the orientational interaction and the translational one, in most cases, leads over time to repulsion between the two objects. The repulsion is qualitatively different for selfaligning objects compared to the more symmetric case of uniform prolate spheroids. The separation between the two objects increases with time t as t1/3 in the former case, and more strongly, as t, in the latter. 

Hydrodynamic Interactions between Two Forced Objects of Arbitrary Shape: I Effect on Alignment
(Submitted on 1 Feb 2015) J. Fluid. Mech. accepted 13Nov 2015,
Abstract: We study the properties and symmetries governing the hydrodynamic interaction between two identical, arbitrarily shaped objects, driven through a viscous fluid. We treat analytically the leading (dipolar) terms of the pairmobility matrix, affecting the instantaneous relative linear and angular velocities of the two objects at large separation. We find that the ability to align asymmetric objects by an external timedependent drive [Moths and Witten, Phys. Rev. Lett. 110, 028301 (2013)] is degraded by the hydrodynamic interaction. The effects of hydrodynamic interactions are explicitly demonstrated through numerically calculated timedependent trajectories of model alignable objects composed of four stokeslets. In addition to the orientational effect, we find that the two objects generally repel each other, thus restoring full alignment at long times.21 pages, 7 figures 

Orientational ordering of colloidal dispersions by application of time dependent external forces, Brian
Moths and T. A. Witten. This paper extends and amplifies the
results of the PRL below. It proves conditions for alignment in
greater detail and shows that the ability to align extends much beyond
the range of these proofs.
Physical Review E
88, 022307 (2013) [15 pages]
. 

Full
alignment of colloidal objects by programmed forcing ,
Brian Moths, T. A. Witten. In this paper, we examine the rotation
exhibited by an asymmetric colloidal object as it falls through a
viscous medium. Given other physical systems where the orientational
degrees of freedom of an ensemble can be intimately controlled by a
uniform external forcing (one example being NMR), we ask if the same is
possible for an ensemble of colloids. The first task one would attempt
might be to align the members of an ensemble. Previous work has shown
that orientational alignment of an ensemble up to rotations about an
axis is possible using constant forcing. In this work, we describe two
methods of aligning by a timedependent but spatially uniform
forcing. We go on to briefly discuss the possible realizations
and limitations of these methods.Physical Review Letters, vol. 110,
Issue 2, id. 028301 (2012) DOI: 10.1103/PhysRevLett.110.028301 

Chiral sedimentation of extended
objects in viscous media
We study theoretically the chirality of a generic rigid object's
sedimentation in a fluid under gravity in the low Reynolds number
regime. We represent the object as a collection of small Stokes spheres
or stokeslets, and the gravitational force as a constant point force
applied at an arbitrary point of the object. For a generic
configuration of stokeslets and forcing point, the motion takes a
simple form in the nearly free draining limit where the stokeslet
radius is arbitrarily small. In this case, the internal hydrodynamic
interactions between stokeslets are weak, and the object follows a
helical path while rotating at a constant angular velocity $\omega$
about a fixed axis. This $\omega$ is independent of initial
orientation, and thus constitutes a chiral response for the object.
Even though there can be no such chiral response in the absence of
hydrodynamic interactions between the stokeslets, the angular velocity
obtains a fixed, nonzero limit as the stokeslet radius approaches zero.
We characterize empirically how $\omega$ depends on the placement of
the stokeslets, concentrating on threestokeslet objects with the
external force applied far from the stokeslets. Objects with the
largest $\omega$ are aligned along the forcing direction. In this case,
the limiting $\omega$ varies as the inverse square of the minimum
distance between stokeslets. We illustrate the prevalence of this
robust chiral motion with experiments on small macroscopic objects of
arbitrary shape.
