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 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 solid particles dispersed in liquid

Self-assembly Colloidal Crystals  Iridescence is from the Bragg’s diffraction  What is the role of entropy in self-assemly?

Entropically driven assembly concentration 49% volume fraction F=U-TS Hard sphere

Entropic Interaction F=U-TS Hard sphere Depletion interaction

Experimental Setup Line-Scanned Optical Tweezers

Interaction Measurement Find the centroids of colloids Measure the separation of two beads 30 minutes video tape -> 10 5 measurements The probability of separation Spatial resolution ~ 30 nm, Energy resolution ~0.05 k B T

Interaction Potential Buffer potential -- optical potential U fd =U optical +U dep

Compared with the theory

Assembly by Depletion Interaction Create “Order” Out of “Disorder”--Vrij

Depletion Interactions between Different Geometry A.D. Dinsmore, A.G. Yodh, and D.J. Pine, Nature, 383, 239 (1996), Langmuir 15, 314 (1999). Control the positions of colloids by surface topology

Making Surface Structures 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.