What is colloids?
Colloids are small particles suspended in a liquid by thermal agitation. Thus, colloidal particles are sufficiently small that collisions with the molecules of the liquid in which they are immersed are sufficient to keep them from sinking or floating under the influence of gravity. This means that colloidal particles are generally smaller than a few micrometers, since larger particles usually sink or float. There is no lower limit to the size of colloids, other than to note that because they are made up of many atoms or molecules, they are generally larger than a nanometer or so. Many everyday products are colloids, including milk, paint, ink, bacteria, viruses, as well as globular proteins. Opals are dried out colloids.
Self assembly of colloids
FIG. 1. Colloidal crystal. Notice that there are both hexagonal and square planes visible, which is consistent with a face-centered cubic lattice.
A suspension of identical colloidal particles can spontaneously self assemble to form a
regularly-spaced lattice. Figure 1 on the right shows such a lattice, a "colloidal
crystal", that has been photographed using an electron microscope. The particles in the
photograph are made from polystyrene, a common commercial plastic. The water in which
the particles were suspended was evaporated to facilitate taking the photograph, but the
same ordered structure of particles apparent in the figure was present before the water
was removed.
The lattice shown in Fig. 1 is a face-centered cubic (FCC) lattice, an arrangement
favored by grocers stacking oranges. Other types of lattices can be made by mixing two
or more particles of different sizes in just the right ratio. However, the kinds of
lattices that can be made with simple spheres is is somewhat limited.
Shaping colloidal particles: Colloidal clusters
FIG. 2. Colloidal clusters of 3 to 10 silica spheres(diameter 900 nm).
Colloids can be any shape in principle. Proteins often fold up into complex oddly-shaped
globules. Many viruses are shaped like rods. Artificial colloids, however, such as those
synthesized for use in paint, are typically spherical, so the repertoire of particles we
can actually make is rather limited.
In order to expand the kinds of structures that can be made with colloids, we have
developed methods for assembling colloids in to small aggregates with well-defined
structures. Figure 2 shows a series of colloidal clusters consisting of 3 to 10
particles. The emulsion encapsulation method we developed for making these clusters
results in a unique configuration of particles for all clusters of a given number: for
example, all 7-mers look exactly the same – there are no isomers. The absence of
isomers has been explained by Eric Lauga and Michael Brenner in
Physical Review Letters 93,
238301 (2004) . You can read more about the clusters in Fig. 2 (and a few more),
including how they are fabricated in the following publications:
Science 301, 483-487 (2003)
[&
Supplementary materials],
Advanced Materials 16, 1204-1208
(2004), and
MRS Bulletin 29, 91-95
(2004).
Hybrid colloidal clusters from bidisperse colloids
FIG. 3. Colloidal clusters made from particles of two different sizes using emulsion encapsulation method.
Working with our collaborators at the Korea Advanced Institute for Science and Technology (KAIST) in Daejeon, South Korea, we have generalized our emulsion encapsulation method in order to form complex clusters made from colloidal particles of two different sizes. Figure 3 shows some of the results. From left to right, the clusters consist of 1, 2, or 3 large particles (2.3 μm diameter, shown in pink) and many small particles (0.23 μm diameter, shown in white). In this photograph, all the spheres are silica, but we made other clusters where the small and large particles were made out of different materials. You can read about this work in JACS 127, 15968-15975 (2005).
More information:
You can obtain more information by contacting David Pineor by
contacting any of his collaborators listed on the right.
You can also read a more complete accounts of our work published in the journal
articles:
Science 301, 483-487 (2003), Advanced Materials 16, 1204-1208 (2004)
, and MRS Bulletin 29, 91-95
(2004).
Collaborators:
The work described on this page is the result of a collaborative effort among a number of
people:
Vinny Manoharan, Harvard University
Mark
Elsessor, American Physical Society
Gi-Ra Yi, POSTECH
Seung-Man
Yang, KAIST
Research Grant
