Self-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?
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
Examples

Molecular level
Intra – protein folding
Inter – lipid assembly
Supramolecular Self-assembly
Involve more than one molecule
Many happens in biological systems.
mitochondria
virus
microvilli
Top down vs. Bottom Up
Brownian Motion
Colloids: nm – mm solid
particles dispersed in liquid
Particles explore accessible phase space by Brownian motion.
Entropy S ~ log W, W is all possible states.
Eg. Spill perfume. Molecules diffuse the whole room.
Self-assembly Colloidal Crystals

Iridescence is from the Bragg’s diffraction

What is the role of entropy in self-assemly?
Entropically driven assembly
Hard sphere
49% volume fraction
concentration
F=U-TS
Entropic Interaction
F=U-TS
Hard sphere
Depletion interaction

h 2
2
U s ( h)   k B Tn s R  (1  )
2

Experimental Setup
Line-Scanned Optical Tweezers
Interaction 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) )
kB T
• Spatial resolution ~ 30 nm,
• Energy resolution ~0.05 kBT
Interaction Potential
Buffer potential -- optical potential
Ufd=Uoptical+Udep
Compared with the theory
Assembly by Depletion Interaction
Create “Order” Out of “Disorder”--Vrij
Depletion Interactions between
Different Geometry
 UA
 UB ~ 2  U A
A.D. Dinsmore, A.G. Yodh, and D.J. Pine, Nature,
383, 239 (1996), Langmuir 15, 314 (1999).
Ge o m e t r ic
T e m p la t e
Control the positions of colloids by surface topology
Making Surface Structures
Co m m e r c ia lly
a
g ra t in g
g r a t in g
f ro m 0 . 3
g la ss s u b st r a t e
a v a ila b le
p it c h e s ra n g e
m ic ro n t o 1 0
m ic r o n .
PM MA la y e r
( > 2 0 0 n m t h ic k )
Press a grating onto PMMA layer while
heating PMMA above its glass temperature.
b
Remove the grating after
PMMA is cool.
AFM pictures of imprint templates
p=0.83 micron
Colloidal Single Crystals Grown on
the Template
A confocal image of a colloidal single crystal
Colloidal Molecules
V.N.Manoharan, M.T.Elsesser, D.J.Pine, Science 301, 483 (2003)
Can we understand molecular
assembly and the role of entropy?
Colloidal molecules as model systems
Entropy driven phase separation in
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.