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.