Nano-soft matter

Hsuan-Yi ChenDept of Physics and Center for Complex Systems, NC

U

Outline

• Motivation: crazy dreams • Self-assembly• Non-equilibrium dynamics• Summary

Motivation: why is nanoscience important or interesting?

Dream: Example:

Crazy dreams (good for publicity, and indeed, this is

what we want!)

We will build nano-machines.

Nano-machines will be intellegent and change (save) our lives.

How realistic is the above statement?

The true lives in nano-world and the hard facts about our crazy dreams

Different dynamics, universal attractive interactions, molecular recognition, mass production, cost/effect……

Back to basic physics of our real world: Intermolecular forces

• All from E&M (some are QM)• Direct Coulomb: 1/r• Dipole in external E field 1/r3

• Dipole-dipole • Dipole-induced dipole, van der Waals 1/r6

• Electrolyte, salt, etc. exp(-r/k)• ** A likes A more than A likes B**. Why??

What can these interactions do for us in systems withmany (say, 100 to 100,000) particles?

Phase transitions and new phases

How to make that kind of structure??

Learn some statistical physics first!

Road to equilibrium: F = U-TS minimumHigh T: large S,

homogeneous phase (ex. Gas)

Low T: small U, ordered phase (ex. Crystal)

Phase transition: (interaction energy) ~ T

(entropy difference)O.Ikala and G. t. BrinkeScience 295 2408 (2002)

AB: energy cost for a pair of A-B neighbors Entropy gain for mixing a pair ofA-B particles ~ kB

Simple systems: Binary fluids

A

B

F = U – TS

Phase separation at kT < O(AB)

A+B

Want to get cool structures?? Use principles of symmetry breaking.Use polymers.

Symmetry breaking : road to special “patterns”

Solidification: isotropic fluid phase anisotropic solid

Rev. Mod. Phys. 52, 1 (1980) Large curvature = large temperature gradient = fast growth

Polymers: material to make “patterns”

homopolymer

coarse-grained view

take thermal fluctuationsinto accountSize: submicron

+

++

+ +

+ + + +

AB diblock copolymer

ABC (linear) triblock copolymer

ABC triblock star

comb

A B

A B C

Block copolymers: designer’s material

AB Interaction between A, B links.

f A Volume fraction of A links.

N Number of links along a chain.

More parameters will be used if we consider more complicated architectures.

Modeling diblock copolymers

Physics Today, Feb. 1999, p32.

What do we expect to get from diblock copolymer melt?

Principles of pattern selection in block copolymer melt

• F = F(elastic) + F(interfacial)• F(elastic) ~ (domain size)2

• F(interfacial) ~ (domain size)-1

• F(homogeneous) ~ fAfBN• Compare free energy per chain for diff

erent phases.

Phases of diblock copolymer

Self-assembly occurs in other systems, too.

Physics Today, Feb. 1999, p32

What we will see when there are three?

Applications: dots

M. Park, C. Harrison, P.M. Chaikin, R.A. Register, and D.H. Adamson

Science 276, 1401 (1997)

Application: Wires

Thurn-Albrecht, J. Schotter, et al.,

Science 290, 2126 (2000)

S.O. Kim, et. al., Nature 424, (2003)

Making patterned surface

Polymer “alloys” designed in nanoscale

triblock pentablock

C.Y. Ryu, et al, Macromolecules, 35 9391 (2002)

Nano-particles on droplets

Nonequilibrium dynamics: make nano-machines

• Nonequilibrium: beyond “partition function” physics.

• What is new for motion in “wet” environment, at nm scale?

• Can we utilize these special features?

Navier-Stokes equation and Reynolds number in nm scale

In cgs units: l~10-7, v~10-7, Re<<1. Strongly overdamped motion.

inertia effect viscous effect

protein folding and protein motors: overdamped, Brownian motion

http://folding.stanford.edu/education/prstruc.html

Science 1999 Nov 26; 286: 1687.

Robert H. Fillingame

I.M. Janosi et al, Eur. Biophys. J. 27, 501 (1998)

Microtubule: non-equilibrium, self-assembled tracks in cells

Filaments in a cell

http://www.accessexcellence.org/AB/GG/cytoSkeleton.html

+ - + - + - +

+2 10 nmRev. Mod. Phys. 69, 1269 (1997)

Nano-machines work on the tracks

Brownian motion is important for life.

R.D. Astumian, Science, 276, 917 (1997)

Application:Particle separation by Brownian motors

Nature 401(1999)

Road to artificial motor

Not very good, not too bad, either.

How are we doing with the artificial motor?

Science 290, (2002)

Nanodevice with natural rotatory motors

A rotatory motor at work

How to make structures like this? (inside a cell)

Need to construct simpler model systems to understand pattern formation in systems of this kind.

Leibler 97: quasi-2d experiments

Kinesin “multimers”.

Kinesins move towards “+” ends. Finally they accumulate near the center.

Taxol: control microtubule length and number

Most of the exp were done without taxol.

Leibler 97: aster and vortex

1. Microtubule length: short = aster, long = vortex. 2. Get vortex at late time due to a “buckling instability”.3. Forming aster is not the only possible route leading to the vortex structure.

Leibler 97: large systems

1. Kinesin concentration has important effects on the resulting pattern. (low=vortices, medium=asters, high=bundles)

2. When two asters overlap sufficiently, they can merge. This process may determine final distance between asters.

Leibler 01: One motor result (still 2d)

Kinesin: + end motorNcd: - end motorVortices only seen in kinesin exp

+ end points outward for Ncd + MT (see MT seed in `h’)

Leibler 01: Two motors result

Motor concentration increases Local MT bundles, poles between bundles

Low kinesin/NcdstarsHigh kinesin/Ncd vortices

Kinesin localized in every other pole(+ poles)

Summary

• Why “nano”?? Why “soft nano”??• Successful story: self-assembled nano

structures.• Failure: real, nano, artificial machines.• One thing for sure: go study physics h

ard.