Nano-soft matter Hsuan-Yi Chen Dept of Physics and Center for Complex Systems, NCU

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/r 3 Dipole-dipole Dipole-induced dipole, van der Waals 1/r 6 Electrolyte, salt, etc. exp(-r/k) ** A likes A more than A likes B**. Why?? What can these interactions do for us in systems with many (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  minimum High 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. Brinke Science (2002)

 AB : energy cost for a pair of A-B neighbors Entropy gain for mixing a pair ofA-B particles ~ k B 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 fluctuations into account Size: 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) ~  f A f B N Compare free energy per chain for different 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, (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 Science 1999 Nov 26; 286: 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

nm Rev. 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 motor Ncd: - end motor Vortices 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/Ncd  stars High kinesin/Ncd  vortices Kinesin localized in every other pole (+ poles)

Summary Why “nano”?? Why “soft nano”?? Successful story: self-assembled nanostructures. Failure: real, nano, artificial machines. One thing for sure: go study physics hard.