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Cite this: Soft Matter, 2012, 8, 4010
www.rsc.org/softmatter EDITORIAL
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View Article Online / Journal Homepage / Table of Contents for this issue
Bridging the gap between hard and soft colloidsDOI: 10.1039/c2sm90031a
Hard sphere suspensions and polymers
can be viewed as two essential represen-
tatives of soft matter, which exhibit
different properties.1,2 Typically, the size
scale of colloids is of the order of microns
whereas it is nanometres for polymers.
While long-range order is achieved in
colloids at relatively small volume frac-
tions, short-range order cannot be at-
tained in polymers even at very high
fractions. The origin of the stress is
entropic in both systems but their
respective dynamics is controlled by
different mechanisms. In polymers, it is
dominated by the elasticity of the chains
and the existence of entanglements that
hinder transverse diffusion, while in
colloids, hydrodynamics and particle
interactions are the key parameters. A
direct consequence is that microstructural
deformation under flow reflects the alter-
ation of particle arrangements in the
latter case and polymer conformation in
the former. Combining these distinct
features represents a formidable challenge
as novel materials with new types of
behaviour can be designed and fabricated,
and their properties explored. For
instance, hard colloidal spheres with
4010 | Soft Matter, 2012, 8, 4010–4013
grafted polymer chains constitute a good
realization of soft colloids where in the
limit of many short chains, the colloidal
nature prevails whereas for a few
extremely long chains the polymeric
response is dominant. This renders the
detailed exploration of the intermediate
world of soft colloids fascinating.
Hard sphere suspensions are arche-
types of many particulate materials. The
excluded volume interaction that charac-
terizes them is the simplest we can
imagine: particles do not interact except
at contact where they undergo a strong
repulsion that prevents interpenetration
or deformation. Nevertheless, hard
sphere suspensions exhibit a very rich
phase diagram, including liquid, crystal-
line and glass phases.3 Obtention of
a given phase depends not only on volume
fraction but also on the conditions of
preparation and parameters like poly-
dispersity, flow and gravity. For instance,
crystallization can be avoided at the
benefit of supercooled or glassy states by
forcing the volume fraction to increase
rapidly. Slightly polydisperse hard sphere
suspensions form glasses when the
volume fraction exceeds a value of about
This journ
fgz 0.58. Glasses are out-of-equilibrium
materials where particles are kinetically
trapped into metastable cages formed by
a small number of neighbours, which
restrict and eventually arrest macroscopic
motion.4 Cages possess an intrinsic elas-
ticity of entropic origin. They are broken
upon application of external stresses
exceeding the so-called yield stress,
causing particles to move past one
another over large distances and leading
to macroscopic flow, which is in general
heterogeneous.5,6,7 The cage elasticity and
yield stress are of the order of 1 Pa or less,
indicating that hard-sphere glasses are
very soft and fragile materials. Another
important feature of glasses is that they
exhibit slow dynamics and aging just like
many other out-of-equilibrium mate-
rials.8 The way hard sphere glasses yield,
flow and age at the mesoscopic and
macroscopic scales are challenging topics
that stimulate intense experimental and
theoretical works.4,9–11
Notwithstanding the conceptual
importance of hard sphere suspensions,
most systems used in real applications are
soft colloids. Softness can be of various
origins.12,13 It can arise from the interac-
tion potential itself, which allows some
degree of compression beyond the effec-
tive radius of the particles. This occurs
naturally as a consequence of the stabili-
zation mechanism—electrostatic or
steric—used to keep the particles apart.
Another source of softness arises from the
particles themselves which can be elastic
and deformable. A non-exhaustive list of
examples includes microgels, emulsion
droplets, vesicles, and hairy particles such
as block copolymer micelles, star poly-
mers, or end-grafted or physisorbed
particles.12,13 In these materials, the upper
bound of the fully disordered glassy
region for hard sphere suspensions, i.e.
the volume fraction at close-packing, can
be easily overcome due to deformability
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and interpenetration, making very large
volume fractions accessible. Jammed
suspensions are generally highly elastic
with a shear modulus of the order of 103
Pa and have a significant yield stress,
which can exceed 102 Pa.14–16 Yielding is
a gradual process that eventually leads to
macroscopic flow;17,18 the latter is often
associated with wall slip, shear banding,
and non local rheology depending on the
type of material, surface interactions, and
confinement.7,19–22 Upon flow cessation,
slow relaxations and various forms of
aging phenomena take place.8,23,24 It is
important to emphasize that softness is
also important in the liquid state.
Suspensions of particles are known to
organize in various ways under flow.25,26
Both flow and material (particle, disper-
sion medium particle interactions) prop-
erties play a key role27,28 and
understanding the consequences of brush
deformation on the flow in soft colloids
remains a challenge.
It is therefore evident that colloids of
varying softness are valuable for a lot of
applications and industrial processes.
Key questions of fundamental and
applied interest concern the nature of the
glass and jamming transitions in soft
colloids, the linear and nonlinear rheo-
logical behaviour of soft suspensions
throughout the entire concentration
range from dilute to jammed and the
possible flow-induced order, the roles of
solvent type and quality, slow dynamics
and aging, the importance of particle
shape and the design of new tailored
architectures. The scope of these topics
extends far beyond the field of soft
colloids since many of them are poten-
tially relevant to other classes of mate-
rials like molecular-glass formers,
metallic glasses and granular materials.
Addressing these questions represents
a fascinating challenge which requires
the skills of statistical and condensed-
matter physicists, chemical engineers,
materials scientists, physicists and
biophysicists as well a combination of
theoretical and experimentally
approaches.
The present themed issue on ‘‘Bridging
the gap between soft and hard colloids’’
fingerprints the wide range of scientific
and technological challenges that have
emerged over the last few years. It reflects
the great richness of the field and the
opportunities for further developments.
This journal is ª The Royal Society of Chemistry
The different contributions of this issue
are grouped in eight gross areas below.
The role of softness on crystallization
Starting from a metastable supercooled
state, the selection mechanisms of poly-
morphs upon crystal nucleation are
different in hard and soft spheres, in the
latter case the bond orientational order
being the key (DOI: 10.1039/
c2sm07007c). Ionic microgel particles
represent one of the main prototypes for
exploring the physics of soft colloidal
particles. Varying their crosslink density
(hence their stiffness) has consequences
on their phase behaviour as the liquid-
crystal phase coexistence region increases
with decreasing particle stiffness (DOI:
10.1039/c2sm06973c). Ultrasoft over-
lapping particles form what are known as
cluster crystals, whose nucleation rates
are accelerated by shear, and which under
flow organize into well-defined quantized
patterns, controlled by particle interac-
tion and flow characteristics (DOI:
10.1039/c1sm06899g).
Particle dynamics in crowded
dispersions
New approaches and techniques have
been developed to probe the local
dynamics and associate it with the meso-
scopic behaviour of jammed particle
suspensions, such as caging and nano-
mechanical properties. Three examples
are presented here: (i) the rotational
motion in a jammed dispersion of parti-
cles consisting of an anti-ferromagnetic
core and a thermosensitive microgel shell
(DOI: 10.1039/c2sm07076f); (ii) the rich
vibration spectroscopy of clusters of
spherical polystyrene latex particles via
Brillouin light scattering (DOI: 10.1039/
c2sm07034k); (iii) and the long-time
tracer diffusion in soft sphere glasses
which is predicted to be more than three
times faster than in hard sphere glasses
(DOI: 10.1039/c1sm06932b).
Predictive microstructural theories for
linear and nonlinear rheology
This broad topic continues to attract the
interest of leading groups in the field
worldwide.Mode coupling theory (MCT)
remains the main predictive tool for
glassy suspensions. Interestingly, below
close packing, the linear viscoelasticity of
2012
core–shell microgels is indistinguishable
from that of true hard sphere systems,
since their soft shells essentially do not
deform (DOI: 10.1039/c2sm07011a).
MCT is shown to form the basis for the
understanding of the nonlinear rheology
of glasses through the development of
schematic constitutive equations where
for example the application of two equal
and opposite step strains leads to
a nonvanishing residual stress signifying
plastic deformation (DOI: 10.1039/
c2sm06891e). Further advances with the
use of MCT showcase the use of different
interaction potentials (nonoverlapping
disks and magnetic dipoles) to study the
structure and viscoelasticity of binary
glasses (DOI: 10.1039/c2sm07010c).
Microscopic models based on Smo-
luchowski theory are shown to be partic-
ularly important in linking
microstructure to rheology. Recent
developments include the prediction of
the pair distribution function and elastic
moduli of jammed microgel suspensions
(DOI: 10.1039/c2sm06940g) and the
rheology of concentrated suspensions of
repulsive particles with hydrodynamic
interactions (DOI: 10.1039/c2sm07187h).
Nonlinear flow phenomena in colloidal
glasses
Yielding and shear banding are impor-
tant phenomena associated with the flow
of glasses, gels and jammed solids.
Whereas they appear in all these systems,
important subtleties and the role of
softness remain in large unresolved. This
prompts a direct phenomenological
comparison of rheology and yielding as
function of volume fraction for different
types of hard and soft colloids (DOI:
10.1039/c2sm07113d). Mesoscopic
modelling accounting for local plastic
events that give rise to global stress
redistribution over the system and hence
macroscopic flow, can predict formation
of permanent shear bands that originate
from local restructuring (DOI: 10.1039/
c2sm07090a). Transient shear banding
upon flow start-up has been studied
experimentally as function of time and
measurement geometry for simple yield-
stress fluids (Carbopol microgels) and
shown to persist throughout the whole
sample or eventually result in complete
fluidization and homogeneous flow,
Soft Matter, 2012, 8, 4010–4013 | 4011
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depending on the applied shear rate
(DOI: 10.1039/c2sm06918k).
Aging and heterogeneous dynamics in
soft glassy materials
When the average relaxation time of
a glassy suspension changes with aging
time and temperature without affecting
the shape of the relaxation spectrum, the
principle of time-temperature superposi-
tion works, as demonstrated for the case
of clays (DOI: 10.1039/c2sm07071e).
Recently developed advanced scattering
techniques of high resolution, such as
real-space analysis of dynamic correla-
tions in conjunction with confocal
microscopy, can be applied to study the
relaxation in glassy suspensions. A
comparative study shows that jammed
soft spheres exhibit far longer-range
correlations compared to hard sphere
glasses, a difference attributed to the
strong internal elasticity of the former
(DOI: 10.1039/c2sm25267h).
Long hairy particles
Besides the deformability that character-
izes a wide range of soft particles such as
microgels, hair interpenetrability at high
fractions and ultrasoft interaction poten-
tial are two additional features of long
hairy particles. Star polymers are a much
studied system in this area, whose poten-
tial is tuned by changing the number of
arms. Hybrid mesoscale simulations
including hydrodynamic interactions are
very powerful in predicting the rheology
as a function of concentration in good
solvent, in agreement with experimental
data (DOI: 10.1039/c2sm07009j). Grafted
particles with different number and size of
long grafts serve as model systems form-
ing quasi-one-dimensional nano-
composites with tunable mechanical
properties (DOI: 10.1039/c2sm06915f).
The role of dispersing medium in the
properties of suspensions
Solvent quality plays a key role on the
interactions of colloidal particles and in
this regard it can serve as ameans to tailor
their properties. This is particularly
important for grafted particles where the
solvent quality for the grafts can be varied
significantly while preserving colloidal
stability; the consequences on the struc-
tural properties and conformation of
4012 | Soft Matter, 2012, 8, 4010–4013
grafts are explored theoretically (DOI:
10.1039/c2sm06836b). Another demon-
stration of the importance of softness,
which leads to novel results with respect
to hard sphere systems, is made by mixing
star and linear polymers. Depletion-
induced star clusters form upon adding
linear polymers, which are irregular and
transient, as suggested by molecular
dynamics simulations and MCT (DOI:
10.1039/c2sm06849d). Nanocomposites,
created by the addition of small particles
to long polymer matrices, have been
studied extensively due to their impor-
tance in enhancingmechanical properties.
However, many important questions
remain unanswered. Here we see how we
can understand the so-called Payne effect,
i.e. the decrease of modulus at high strain
amplitudes, in composites made of inor-
ganic particles added to elastomers, by
quantitatively accounting for interpar-
ticle contacts (DOI: 10.1039/
c2sm06885k). Oligo-tethered nano-
particles suspended in small polymers
represent a versatile model system for
investigating jamming transitions and
associated rheological phenomena, which
can be discussed in the framework of the
soft glassy rheology model, a key alter-
native to MCT (DOI: 10.1039/
c2sm06889c). Dispersing particles in
liquid crystalline solvent has a great
impact on their relaxation dynamics and
aging behaviour as a consequence of the
solvent anisotropy (DOI: 10.1039/
c2sm06986e).
Developments in synthetic and surface
chemistry
As is typical in soft matter research, this
field has benefited a great deal from the
synergy of chemistry, physical experi-
ments and theoretical rationalization.
Undoubtedly however, the advances in
synthesis and characterization provide
new directions for fabricating novel
materials with unique properties that can
be tailored at molecular level and open
new opportunities for investigating
further the physics and applications of
complex colloids. Two such cases are
included in this issue and demonstrated
the limitless opportunities in this exciting
field. A review of the formation of crys-
talline one- and two-component colloidal
monolayers shows the possibilities for
applications in nanolithography and sets
This journ
the challenges ahead (DOI: 10.1039/
c2sm06650a). Preparation of metallic
nanoparticles grafted with microgels is
another important development offering
tunable colloidal systems with tunable
optical response (DOI: 10.1039/
c2sm06396k).
Although a great number of investi-
gations on hard (in particular) and soft
colloids exist, only in the last few years
they have been put into context and the
field has emerged as very important in
soft matter. Hence we believe that this
themed issue is timely. Recent develop-
ments have been triggered by significant
advances in theoretical modelling and
simulations as well as experimental
techniques. We hope that the readers of
this issue will appreciate the large
potential impact of this emerging field
and the ample possibilities for further
progress. It is a most exciting area for
research in this branch of soft matter
physics and technology.
Dimitris Vlassopoulos
Michel CloitreaFORTH, Institute of Electronic
Structure & Laser, Heraklion 71110,
Crete, GreecebUniversity of Crete, Department of
Materials Science & Technology,
Heraklion 71300, Crete, GreececESPCI ParisTech, Mati�ere Molle et
Chimie (UMR ESPCI-CNRS 7167), 10
rue Vauquelin, 75005 Paris, France
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