Self-Assembly of Colloidal ParticlesSelf-Assembly of Colloidal Particles

田安庭 ,孫文瑜 ,林耿慧中央研究院物理所

What is self-assembly?What is self-assembly?

Characteristics-- Spontaneous

-- Increase the organization

-- from Molecule to Galaxy

-- Can be dynamical or static (Equilibrium)

-- Key to nanotechnology

the best way to manufacture things at nano-scale

ExamplesExamples

Molecular level Intra – protein folding Inter – lipid assembly

Supramolecular Self-assemblySupramolecular Self-assembly

Involve more than one moleculeMany happens in biological systems.

mitochondria virus microvilli

Top down vs. Bottom UpTop down vs. Bottom Up

Brownian MotionBrownian Motion

Particles explore accessible phase space by Brownian motion.

Entropy S ~ log is all possible states.

Eg. Spill perfume. Molecules diffuse the whole room.

Colloids: nm – mm solidparticles dispersed in liquid

Self-assembly Colloidal CrystalsSelf-assembly Colloidal Crystals

Iridescence is from the Bragg’s diffraction

What is the role of entropy in self-assemly?

Entropically driven assemblyEntropically driven assembly

concentration

49% volume fraction

F=U-TS

Hard sphere

Entropic InteractionEntropic Interaction

F=U-TSHard sphere

Depletion interaction

Us(h) 2kBTnsR 2(1

h

)2

Experimental SetupExperimental Setup

GalvanometerMirror

Telescope

Nd:YLFLaser

InvertedMicroscope

CCDCamera

=1053 nm

Line-Scanned Optical TweezersLine-Scanned Optical Tweezers

Interaction MeasurementInteraction Measurement

• Find the centroids of colloids• Measure the separation of two beads• 30 minutes video tape -> 105 measurements• The probability of separation

P(r)exp( U(r)kBT

)

• Spatial resolution ~ 30 nm, • Energy resolution ~0.05 kBT

Interaction PotentialInteraction Potential

Buffer potential -- optical potential

Ufd=Uoptical+Udep

Compared with the theoryCompared with the theory

Assembly by Depletion InteractionAssembly by Depletion Interaction

Create “Order” Out of “Disorder”--Vrij

Depletion Interactions between Depletion Interactions between Different GeometryDifferent Geometry

UB~2UA

UA

A.D. Dinsmore, A.G. Yodh, and D.J. Pine, Nature, 383, 239 (1996), Langmuir 15, 314 (1999).

Geometric Template

Control the positions of colloids by surface topology

Making Surface StructuresMaking Surface Structures

glass substrate PMMA layer

grating

(>200 nm thick)

Press a grating onto PMMA layer whileheating PMMA above its glass temperature.

aCommercially availablegrating pitches rangefrom 0.3 micron to 10 micron.

Remove the grating afterPMMA is cool.

b

AFM pictures of imprint templates

p=0.83 micron

Colloidal Single Crystals Grown on Colloidal Single Crystals Grown on the Templatethe Template

A confocal image of a colloidal single crystal

Colloidal MoleculesColloidal MoleculesV.N.Manoharan, M.T.Elsesser, D.J.Pine, Science 301, 483 (2003)

Can we understand molecular Can we understand molecular assembly and the role of entropy?assembly and the role of entropy?

Colloidal molecules as model systems

Entropy driven phase separation in Entropy driven phase separation in biology?biology?

The DNA is separated from the rest of the cytoplasm in a prokaryote - but there is no nuclear membrane. Can this be an entropic effect due to the depletion attraction?

Goodsell, David S. Our molecular nature : the body's motors, machines and messages, New York : Copernicus, 1996.