non-equilibrium computer experiments of soft matter systems arash nikoubashman institute of...

Non-Equilibrium Computer Experiments of Soft Matter Systems

Arash Nikoubashman

Institute of Theoretical PhysicsVienna University of Technology

[email protected]

Table of Contents

• Introduction• Simulation Technique• Flow Properties of Cluster Crystals• Cluster Crystals under Shear Flow• Cluster Crystals under Poiseuille Flow• Conclusions & Outlook• Appendix

Introduction

What is Soft matter?• Mesoscopic particles (1nm – 1mm) dispersed in

an atomic solvent

Introduction


an atomic solvent

• Everyday soft materials:• Blood

Introduction


an atomic solvent

• Everyday soft materials:• Blood• Paint

Introduction


an atomic solvent

• Everyday soft materials:• Blood• Paint• Milk

Introduction


an atomic solvent

• Everyday soft materials:• Blood• Paint• Milk• Ice Cream

Introduction

…and why are these substances soft?• Elastic constant G for a simple cubic crystal1:

G = 1/v eF’’(r=a)

[1] C. N. Likos, Phys. Rep. 348, 267 (2001)

Atomic Crystal Colloidal Crystal

e 10-1 – 10 eV kBT ≃ 1/40 eV

F Irrelevant Irrelevant

v Å3 = 10-30 m3 10-18 – 10-21 m3

Gcolloidal/Gatomic = 10-12 – 10-9

Introduction

Why is soft matter out of equilibrium interesting?

Introduction

It is omnipresent in our daily lives!

Introduction

It is omnipresent in our daily lives!• Cellular transport2

[2] Medalia et al., Science 298, 1209 (2002)

Introduction

It is omnipresent in our daily lives!• Cellular transport• DNA sequencing3

[3] M. Zwolak and M. Di Ventra, Rev. Mod. Phys. 80, 141 (2008)

Introduction

It is omnipresent in our daily lives!• Cellular transport• DNA sequencing• Blood flow4

[4] Pan et al., Microvasc. Res. 82, 163 (2011)

Introduction

It is omnipresent in our daily lives!• Cellular transport• DNA sequencing• Blood flow• Microfluidics5

[5] T. M. Squires and S. R. Quake, Rev. Mod. Phys. 77, 977 (2005)

Introduction

It is omnipresent in our daily lives!• Cellular transport• DNA sequencing• Blood flow• Microfluidics• Paint

Introduction

It is omnipresent in our daily lives!• Cellular transport• DNA sequencing• Blood flow• Microfluidics• Paint• Oil recovery

Introduction

Interesting flow properties!• Shear thickening6

[6] http://www.youtube.com/watch?v=KL8--cmew3k

Introduction

Interesting flow properties!• Shear thickening• Shear thinning7

[7] http://www.youtube.com/watch?v=pes1Ju1Cl8o

Introduction

Interesting flow properties!• Shear thickening• Shear thinning• Ferrofluidics8

[8] http://www.kodama.hc.uec.ac.jp/spiral/

… and much more

Table of Contents


Simulation Technique

Computational task• Simulation of complex

fluids in and out of equilibrium• Take hydrodynamic

interactions (HI) of solvent into account9

[9] http://iffwww.kfa-juelich.de/www/Applets/iMPC/


“Naïve” approach: pure MD simulations • Pro:• Straight-forward implementation• Atomistic simulations• Contra:• Large disparity in length- and timescales between

solute and solvent particlesComputationally expensive, O(N2)


“Our” approach: Multi-Particle Collision Dynamics10 • Pro:• Hydrodynamics fully resolved• Thermal fluctuations preserved• Many different flow fields possible• Can be easily integrated into existing MD codes• Very fast and scalable algorithm, O(N)• Contra:• Coarse grained fluid

[10] A. Malevanets & R. Kapral, J. Chem. Phys. 110, 8605 (1999)

Shear flow Poiseuille flow


• Flow profile not superimposed, but self-emerging (through the appropriate boundary conditions)• Thus, we can induce:• Wall-Slip• Nonlinear velocity profiles• …

Table of Contents


Flow Properties of Cluster Crystals

• We study particles interacting via GEM potential:

• Potential is:• Purely repulsive• Bounded

Partial and full particle overlap is possible

Flow Properties of Cluster Crystals

GEM crystals have peculiar equilibrium properties• Clustering• Density independent lattice constant• Activated hopping

Cluster Crystals under Shear Flow

• What happens out of equilibrium?• Let’s shear the system11!

soft

hard ?

[11] A. Nikoubashman, G. Kahl and C. N. Likos, Phys Rev. Lett. 107, 068302 (2011)


• Shear destroys crystalline order• System melts and array of strings emerges!

soft

hard


• What if we shear even stronger?

soft

hard ?


• Particles can escape from their string• System destabilizes and melts completely

soft

hard


• Potential exerted by a string of GEM particles


• Free volume decreases rapidly• Fluid resistance grows, viscosity increases

Free volume of the system as a function of shear-rate


• Free volume decreases rapidly• Fluid resistance grows, viscosity increases

Shear-stress as a function of shear-rate

Table of Contents


Cluster Crystals under Poiseuille Flow

Expose cluster crystal to Poiseuille flow12

• Velocity profile of pure solvent:

• Local shear rate:

How does the crystal react?

[12] A. Nikoubashman, G. Kahl and C. N. Likos, Soft Matter, DOI:10.10139/c1sm06899g (2012)


Scenario I• String-formation close to

the walls• Crystalline layer(s) at the

center of the channel

Scenario II• String phase is global, no

microphase separation!

Crystalline layers act on strings as external potential

Strings break up into clumps

Thick crystalline slab flows

Presence of crystal flattens velocity profile


Flow strongly affected by GEM crystal

Velocity profile of the liquid in the presence of the GEM crystal

Particle flux of solute particles. Arrows indicate when the first layer melts


Flow quantization

Plateau height of the plug flow pattern Width of the flat part of the velocity profile

Table of Contents


Conclusions & Outlook

Conclusions•Soft matter in and out of equilibrium is ubiquitous in our daily lives•MPCD technique is a suitable means for studying it

Outlook•Monomer resolved simulations of cluster crystals•Polymeric networks under flow

Thank you for your attention!

The End

Table of Contents


Appendix

Flow dynamics: two step process1. Streaming step:

Appendix

Flow dynamics: two step process1. Streaming step:2. Collision step:

Appendix

• String-formation independent of initial configuration• Can we exploit this to accelerate crystallization?

Appendix

Yes, shear facilitates the crystallization process10!

Color coded density profiles. Top half: unsheared system, lower half: presheared system

[10] A. Nikoubashman, G. Kahl and C. N. Likos, Soft Matter, DOI:10.10139/c1sm06899g (2012)

Appendix

non-equilibrium computer experiments of soft matter systems arash nikoubashman institute of...

Documents

shear flow slide

blood slide

shear flow cluster crystals

soft hard slide

possible slide

shear flow particles

hopping slide

de slide