tItle aNd abstRaCts
New ColloIds
RRI
19th-21st July 2015
talks
SUNDAY JULY 19th 2015
9:00 - 10:00 am: David Pine, NYU, New York
Title: DNA coated colloids with and without directional interactions
10:30 - 11:30 am: Ganesh Subramanian, JNCASR, Bangalore
Title: The role of weak Brownian motion in dilute (anisotropic) suspension rheology – Connections to
the coil-stretch hysteresis.
Abstract: Einstein (1906) first calculated the viscosity of an infinitely dilute suspension of spherical
particles as eff =  + (5/2), accounting for the enhancement due to the suspended particulate
phase, and with 5/2 now known as the Einstein coefficient. The analogous calculation for a
suspension of anisotropic particles is considerably more complicated. For the simplest anisotropic
geometry, that of a spheroidal particle, Jeffery (1922) showed the existence of closed orbits for the
spheroid orientation vector in simple shear flow. The existence of these orbits leads to a rheological
indeterminacy in that the dilute suspension has no unique long-time rheological response; the
particular response obtained, either steady or unsteady, depends sensitively on the initial orientation
distribution.
We have recently considered the role of inertia, both of the fluid (characterized by the micro-scale
Reynolds number Re) and of the particle (characterized by the Stokes number St), in resolving the
aforementioned rheological indeterminacy. Both fluid and particle inertia drive an irreversible drift of
the orientation vector across Jeffery orbits, rendering one or more of the Jeffery orbits stable at the
expense of all others. For prolate spheroids, and for oblate spheroids with aspect ratios greater than
about 0.14, a unique Jeffery orbit is stabilized, eliminating the rheological indeterminacy, and
allowing one to arrive at a unique steady-shear rheology as a function of the spheroid aspect ratio. For
oblate spheroids with aspect ratios less than 0.14, the inertial drift is bi-directional and the resulting
stabilization of a pair of Jeffery orbits still leads to a rheologically indeterminate scenario due to the
dependence of the basins of attraction, of these orbits, on the initial orientation distribution.
Remarkably, this indeterminacy is only eliminated, over asymptotically long times, by weak
stochastic fluctuations, of a thermal or hydrodynamic origin, that set up a unique distribution across
Jeffery orbits. We analyse the case of thermal (Brownian) fluctuations in some detail.
The analysis above is of interest since it involves a Jeffery-orbit-aligned non-orthogonal coordinate
system, and the solution for the orientation distribution, in this coordinate system, reduces to a onedimensional drift-diffusion equilibrium in a bi-stable potential. This insight allows one to connect
both the transient hysteretic dynamics, and the final equilibrium, to the well known Coil-stretch
transition in polymer physics. The two Jeffery orbit equilibria (the so-called tumbling and spinning
modes) are the equivalents of the coil and stretched states of a polymer molecule, and the particle
aspect ratio plays the role of the Deborah number. The combination of inertia and weak Brownian
motion allows us to again calculate the unique steady-shear rheology of a dilute suspension, as a
function of Re Per (Per being the rotary Peclet number). It is shown that there is a large range of
aspect ratios over which micro-scale inertia leads to an overall shear-thickening rheology for a dilute
suspension.
11:30 am - 12:30 pm: Surajit Dhara, University of Hyderabad, Hyderabad
Title: Topological defect mediated self-assembly and structural transition of liquid crystal colloids
Abstract: Topological defects observed in liquid crystals are created during the phase transition from
the isotropic phase. They can also be produced in a controlled way by introducing foreign particles
into the uniformly aligned liquid crystal. Due to elastic distortion around each particle, nematic
colloids experience long-range forces, which opens new directions in colloidal assembly. In this talk, I
will present some of our recent results on the topological defects and directed self-assembly of various
foreign particles in liquid crystals. We study the influence of elasticity on the defects and interparticle
separation of both nematic (N) and smectic-A (SmA) colloids. Our results on nematic colloids provide
direct verification of recent theoretical prediction. We prepared several two-dimensional colloidal
crystals using laser tweezers. We show that the N-SmA phase transition has a strong influence on the
pairwise colloidal interaction and is responsible for the structural transition of two-dimensional
colloidal crystals.
2:30 - 3:30 pm: Snigdha Thakur, IISER, Bhopal,
Title: Biomimetic behaviour of synthetic particles
Abstract: Self-propelled motion involving conversion of chemical energy to mechanical energy
internally is widespread in nature. Examples include biological motors which play essential roles in
the transport and synthesis biochemicals in the cytoplasm and in cell motility. In addition to these
biochemical motors, synthetic molecular motors have been designed that use chemical, light, or other
energy sources to perform directed motion. Model for one such class of synthetic motors, where the
motion does not rely on the conformational change will be discussed. This class of motors includes
electrochemically synthesized striped bimetallic nanorods and synthetic catalytic molecules tethered
to inactive particles. We also probe the collective behaviour of such motors.
The chemo-mechanical propulsion if not limited to particle-like elements, rather there are many
instances where such conversion occurs on filaments. We study the dynamics of such active semiflexible filament. We show that the filament exhibits three distinct type of motion, namely,
translational, snaking and rotation as the rigidity of the filament decreases.
4:00 – 5:00 pm: Abhrajit Laskar, IMSc Chennai and Raj Kumar Manna, IITM,
Chennai
Title: Flow-induced propulsion, aggregation, and self-assembly in chains made of active colloids.
Abstract: Colloidal particles can be rendered active by inducing slip velocities on their surface. A
complete theory of hydrodynamic interactions between active spherical colloids in slow viscous flow
has recently been presented by Singh, Ghose and Adhikari. Using this theory, we construct equations
of motion for the dynamics of chains of active colloidal particles, and study their flexible and stiff
limits.
Flexible chains are linearly unstable above threshold activity levels and propel in their self-generated
flow. The can be used as attachable motors for the transport of passive colloidal particles. Three
different activity regimes are observed.
Stiff chains approximate apolar active rods. In suspension, lateral hydrodynamic attractions in
extensile filaments lead, independent of volume fraction, to anisotropic aggregates which translate
and rotate ballistically. Lateral hydrodynamic repulsion in contractile filaments lead, with increasing
volume fractions, to microstructured states of asters, clusters, and incipient gels where, in each case,
filament motion is diffusive.
Our results demonstrate that the interplay of active hydrodynamic flows and excluded volume
interactions provides a generic nonequilibrium mechanism for hierarchical self-assembly and
transport in active soft matter.
MONDAY JULY 20th 2015
9:00 - 10:00 am: David Pine, NYU, New York
Title and abstract: Random Organization: Reversibility in non-Brownian suspensions
10:30 - 11:30 am: Rajesh Ganapathy, JNCASR, Bangalore
Title: Insights into the Structural Glass Transition from New Colloids Experiments
Abstract: Despite decades of intense research, it is far from clear whether the transformation of a
liquid into a glass is fundamentally thermodynamic or dynamic in origin. The root cause for this
prevailing state of affairs is primarily two-fold: 1. the liquid falls out of equilibrium well before the
putative “ideal glass transition”, a finite temperature thermodynamic phase transition is reached. 2.
Although simulations and colloid experiments allow access to real-space dynamics, currently not
possible with atoms and molecules, the limited dynamic range presently accessible in these studies
does not help distinguish between competing frameworks of the glass transition. A recently proposed
approach attempting to alleviate these difficulties is to freeze a subset of particles in the equilibrium
configuration of the liquid and probe its influence on the statics and dynamics of the free particles.
For suitable pinning geometries, this approach can not only enable identify subtle structural
correlations in the liquid but also non-trivial dynamic ones. In my talk, I will describe recent
experiments on colloidal glasses wherein a configuration of particles is pinned using holographic
optical tweezers. We directly checked the key predictions of two prominent and competing
frameworks of the glass transition, namely, Dynamical Facilitation – a purely kinetic approach, and
the Random First-Order Transition (RFOT) Theory – a thermodynamic approach. Our findings tilt in
favor of RFOT.
11:30 am -12:30 pm: Guruswamy Kumaraswamy, NCL, Pune
Title: Macro-assemblies from bonding colloids
Abstract: We demonstrate that ice-templating can be used to create flexible "bonds" between colloids.
Colloidal assemblies prepared in this manner have remarkable properties. For example, we show that
ice-templating allows us to create macroporous monolithic structures that are essentially inorganic (>
90% by weight of inorganic) and yet that can recover elastically after 90% compression. Even more
unusually, the modulus of these structures increases with temperature, suggesting that the origin of the
mechanical properties in these ice-templated structures is entropic in origin. We can use the same
technique to create linear and branched colloidal-analogues of polymers and can systematically vary
the flexibility of these colloidal chains.
2:30 pm - 3:30 pm: Ambarish Ghosh, IISc, Bangalore
Title: Magnetic nanoswimmers
Abstract: My talk will focus on a system of helical nanostructures that can be actuated with small
magnetic fields to move through fluidic media similar to various bacterial species. It has been possible
to achieve extremely well controlled motion with these nanostructures, which can be used in sensitive
microrheological measurements. It is possible to achieve autonomous motion in these objects using
magnetic fields alone, which may be of interest to the active matter community. Interestingly, these
structures can be engineered to have strong chiro-optical response, which can affect their dynamics in
interesting ways.
4:00 -5:00 pm: Rema Krishnaswamy, IISc, Bangalore
Title: Yielding of a dense polydisperse 2D suspension under periodic shear
Abstract: Yielding under oscillatory shear is commonly employed to test the mechanical strength of
viscoelastic materials in material science, whether in 3D bulk or on interfacial layers formed at fluid
interfaces. In many soft jammed materials, the yielding occurs above a strain amplitude y leading to
a transition from a solid-like to a fluid-like behavior with loss modulus (G”) > elastic modulus (G’).
Experimental attempts to obtain a microscopic point of view of this rheological response in particulate
systems have revealed that below the yield point, after the transient, all the particles return to the same
position at the end of the periodic drive whereas the particle motions becomes diffusive at γ >
γ_y . This reversible to irreversible transition [1] has also been identified as a second order
nonequilibrium phase transition in the universality class of directed percolation or conserved directed
percolation model. Nevertheless, an alternative explanation has also been proposed where this
irreversibility is linked to the onset of chaos [2]. Another point that remains unanswered in the light
of this on going debate [3] is whether a truly reversible state exists below the yield point in jammed
systems where interparticle interactions cannot be ignored.
We examine some of these aspects in a new class of amorphous solids: Langmuir monolayer of an
insoluble surfactant formed at the air/water interface. At high surface concentration of the
amphiphiles, the monolayer consisting of domains of Liquid Condensed phase dispersed in a Liquid
Expanded phase form a dense, polydisperse two dimensional (2D) suspension. Through particle
imaging velocimetry we correlate the microscopic motion of the domains with macrosocopic
mechanical properties of the monolayer near the yield strain under periodic shear. On examining the
domain motion over a few shear deformation cycles i) a reversible to irreversible transition can be
identified at the yield strain with the dominant nonlinear higher harmonics (I3/I1) as the order
parameter for the transition ii) the relaxation times extracted from the transient behaviour of the loss
modulus as well as I3/I1 diverge near the yield point; also pointing to a non-equilibrium critical
phenomena in 2D. Surprisingly on examining the microscopic domain dynamics over larger number
of shear cycles, a ballistic motion is observed both below and above the yield point reminescent of
the grain boundary dynamics observed recently in a colloidal polycrystal [4] and brings into question
the relevance of transition discussed above. We will present these results and the analogy of the
present study with other sheared jammed systems [5,6,7].
References:
1] L. Corte, P. M. Chaikin, J. P. Gollub and D. J. Pine, Nature Physics 4, 420 (2008)
2] I. Regev, T. Lookman and C. Reichhardt, Phys. Rev. E, 88, 062401 (2013).
3] E. Tjhung and L. Berthier, Phys. Rev. Lett. 114, 148301 (2015).
4] E. Tamborini, L. Cipelletti, and L. Ramos, Phys. Rev. Lett. 113, 078301 (2014).
5] E.D. Knowlton, D. J. Pine and L. Cipelleti, Soft Matter, 10 6931 (2014)
6] K. H. Nagamanasa, S. Gokhale, A. K. Sood and R. Ganapathy Phys. Rev. E 89 062308 (2014).
7] N. C. Keim and P. A. Arratia, Phys. Rev. Lett. 113, 078301 (2014).
TUESDAY JULY 21st 2015
9:00 -10:00 am: David Pine, NYU, New York
Title and abstract: Colloidal surfers
10:30 - 11:30 am: Sriram Ramaswamy, TIFR, Hyderabad
Title: Active colloids
Abstract: Colloidal particles whose surface is patterned with a catalyst display self-propelling
behaviour when surrounded by the chemicals whose reactions the catalyst promotes. The character of
the motility depends on the symmetries of the catalytic coat and the local interaction of reactant and
product with the colloid surface. I will present an update on work done in collaboration with
Suropriya Saha and Ramin Golestanian on design principles for gradient-responsive active colloids. I
will talk about single-particle behaviour in externally imposed chemical fields, the complex dynamics
of pairs and the resulting rich collective behaviour.
11:30 - 12:30 pm: Prerna Sharma, IISc, Bangalore
Title: Chirality induced assembly of crenellated colloidal membranes
Abstract: Thin two-dimensional sheets often develop rippled edges through buckling instabilities
which occur on application of external/internal stresses. We show a novel mechanism of generating
rippled edges in colloidal membranes using chirality of their constituents. Chiral symmetry breaking
in colloidal membranes composed of mixtures of left handed and right handed particles leads to the
formation of point defects at the edge of the membrane with each defect separating ripples of
opposite chirality. In addition to spontaneous formation of these defects, optical tweezers can also be
used to imprint them on the edge. We measure the interaction forces between a pair of defects on the
edge and show how their magnitude can be tuned with stoichiometric ratio of left-handed to righthanded particles. Taken together, our observations demonstrate that molecular chirality offers a
robust pathway to self-assemble crenellated sheets where geometric properties of the ripples can be
tuned with remarkable control.
2:30 – 3:30 pm: Ayan Banerjee, IISER, Kolkata
Title: The magic of the transient: Microbubbles in optical tweezers
Abstract: Bubbles are 'normally' un-trappable in optical tweezers (OT), just because they have
refractive index lower than the surrounding fluid medium, so that they are typically pushed away from
the high intensity region of the trapping laser beam. While they can be randomly produced while
trapping micro-objects due to heating of the trapping beam, we have developed several techniques of
controllably creating microbubbles in OT by using: a) an absorbing soft material in an aqueous
dispersion that leads to enhanced heating and the eventual formation of a microbubble when the
material is in contact with the trapping laser, and b) an absorbing substrate in the trapping chamber
where bubble nucleation centers can be created by heating caused due to local plasmons. The
nucleation of a bubble leads to interesting convective flows around it, principally as a result of
Marangoni convection that arises due to the surface tension gradient along the bubble. The convective
flows lead to a variety of effects that we will discuss in the talk. Firstly, the bubble is 'trapped' around
the focal spot of the trapping laser due to the opposing fluid flows around it, and we experimentally
demonstrate that the trapping forces that arise due to such thermophoretic trapping are much stronger
than that produced on dielectric particles by the intensity gradient of the trapping laser (few nN
against the fractions of nN produced by light intensity gradients). In addition, the microbubble, once
trapped, can be manipulated at high velocities (mm/s - which is typically around 1-2 orders of
magnitude higher than the bubble size) by moving the laser spot. We demonstrate a number of
applications using the microbubbles. These include: 1) assembly of microparticles on the surface of
the bubble which can then be manipulated controllably, and even deposited at a region of choice, 2)
exploring the critical temperature - or the so-called phase explosion point of water where it ceases to
exist even as a superheated liquid, 3) manipulation of the trajectories of microparticles by growing
two bubbles adjacent to each other, and finally, and most importantly, d) the formation of permanent
patterns of materials by exploiting the flows around a bubble and the fact that it can be manipulated
by moving the laser beam. The lithographic technique that we have established has been used in a
host of applications that range from site specific catalysis that could lead to catalytic chips, to
patterning conducting polymers for fabricating micro-circuits. We shall also talk about several
research plans that we have with the microbubbles, which seem to offer a new domain of exciting
research using OT.
4:00-5:00 pm: Soumyajit Roy, IISER, Kolkata
Title: Soft Oxometalates (SOMS): What are they? Where are we?
Abstract: The field of polyoxometalates [1], a class of metal-oxide based neutral or anionic or cationic
clusters, in recent times, has entered a new arena of ‘soft’-supramolecular interactions, crossing the
molecular regime of covalent bonds. Such soft structures show soft-matter properties (like scattering,
shear under soft forces, etc.) and are proposed to be called ‘soft’ oxometalates (SOMs). [2] Snthesis of
soft-oxometalates will be discussed. For instance, using ultralow concentration an interesting
phenomenonlogy has been recently observed where simple phosphomolybdate Keggin type
oxometalates show spontaneous and intriguing structure formation, like those of peapods. [3]
However for the purpose of application an ability to direct structure formation in mesoscopic length
scales, is needed. This problem can be circumvented by a method of ‘colloidal casting’ [4,5] and their
subsequent patterning using thermo-optic tweezers. [6]. The potential application of these systems as
catalysts will also be touched upon. [7,8] We will also discuss a recently developed SOM based active
motors and how we make them move. All these aspects will be discussed with relevant examples to
demonstrate structure formation with oxometalates in mesoscopic regime.
Figure 1 – A representative SOM structure.
REFERENCES
[1] Long, D.-L., Tsunashima, R., Cronin, L., Angew. Chem. Int. Ed., 49 (2010) pp 1736-1758;
[2] Roy, S. Comments Inorg. Chem. 32 (2011) pp 113 - 126. See also: S. Roy, (Ed) SOMs: Design,
Application & Perspectives, J. Mol. Eng. Mater., (2014) (in press).
[3] Roy, S., et. al., Langmuir, 23 (2007) pp 5292-5995.
[4] Roy, S., et. al., Langmuir, 23 (2007) pp 399-401.
[5] Roy, S., et. al., Langmuir, 24 (2008) pp 666-669.
[6] Roy, B., et. al., Langmuir, 29 (2013) pp 14733-14742.
[7] Chen, D., et. al., Dalton Trans. 42 (2013) pp 10587-10596.
[8] Sahasrabudhe, A. & Roy, S., J. Mol. Eng. Mater., (2014) doi:10.1142/S2251237314400024.