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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,

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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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