We describe entropic depletion-induced assembly experiments to synthesize
novel colloidal crystals. Our samples are typically a mixture of large and small
species suspended in water or in an organic liquid. The larger particles are
spherical colloids with diameter ranging from 0.4 to 3 microns, and its type
range from polymer-based spheres such as polymethylmethacrylate (PMMA),
polystyrene (PS) and silica, to semiconductors such as ZnS, to novel
polymer-based spheres that have hollow interiors. The small species can be a
polymer (e.g. PS polymer), a micelle (e.g. SDS micelles), a dendrimer, or a very
small colloidal particle. Typically an optical
grating template is bonded to one wall of our microscope slide cells to select
particular colloidal crystal types.
We have investigated the entropic self-assembly of colloidal spheres on periodic
patterned templates [1,2]. A variety of two-dimensional structures with
quasi-long-range order are observed to form on templates with one- and
two-dimensional periodic structure. In particular, on a template commensurate
with an FCC (100) plane, entropic forces induced by non-adsorbing polymers form
an FCC crystal more than 30 layers thick without random stacking defects [3].
We have also begun particle synthesis experiments to achieve samples of
monodisperse colloidal suspensions of the types mentioned above. So far our
successes in this endeavor are as follows. (1) We could modify a published
technique [4] to synthesize 140 nm and 400 nm ZnS particles with size-dispersity
< 5%. In principle this same procedure can be applied to synthesize a range of
similar sulfides, e.g. CdS, SnS, PbS, MnS. (2) We have prepared monodisperse
particles that have a water-swollen core (precursor to hollow sphere particles)
with particle sizes in the 550-650 nm range [5,6]. Initial success of their
assembly by depletion techniques has been achieved in our laboratory [7]. (3) We
have also succeeded in synthesizing 100 nm, 200 nm, and 600 nm
monodisperse (<10%) magnetic particles following a literature technique [8,9].
We will describe experiments to measure the photonic band structure of some of
these crystals using a spectrophotometer set up in our laboratory, which can
perform angle- and wave length resolved reflection and transmission experiments
on our colloidal crystalline samples.
Sanyal, S., Zhang, J., Work, W.J., Lin, K.H., Yodh, A.G., Colloidal Assembly in Entropically Driven, Low-Volume-Fraction Binary Particle Suspensions, Fifth Microgravity Fluid Physics and Transport Phenomena Conference, NASA Glenn Research Center, Cleveland, OH, CP-2000-201470, pp. 278-313, August 9, 2000.