Controlled Assembly of Jammed Colloidal Shells on Fluid Droplets
Zach Wissner-Gross (March 9, 2009)
Controlled assembly of jammed colloidal shells on fluid droplets
Anand Bala Subramaniam, Manouk Abkarian, and Howard Stone
Nature Materials, 2005, 4, 553-556
Soft matter keywords
Adsorption, self-assembly, surface tension
Stone and coworkers use hydrodynamic focusing in PDMS microchannels to form what they call "colloidal armor" around a gas or liquid bubble. They accomplish this by flowing two colloid suspensions from either side into the path of a central channel containing the liquid/gas that will ultimately become the core of the colloid shell (Figure 1). Fluorescent polystyrene beads were chosen here as the colloid.
The authors also noted several very interesting properties of the assembly. First was that the colloidal shells were "jammed," as stated in the paper's title, meaning that the colloidal particles did not diffuse around the surface, instead exhibiting solid or crystalline behavior with a few predictable point defects. This allowed them to record a rather fascinating image in which they created a bubble whose armor consists of two hemispheres of different colors (Figure 2). Also notable was the fact that only certain particles were able to become a part of the colloidal armor. This will be discussed in greater detail below.
What is novel about this shell synthesis method was, as the authors say, that it "allows an unprecedented degree of control over armour composition, size and stability." I find multicomponent shell synthesis (Figure 2) to be particularly exciting, as it can lead to the creation of microparticles expressing multiple antigens in well-defined and locked positions for drug delivery and other applications.
Soft matter discussion
As I stated in the summary, only certain colloidal particles were integrated into the shells, and there's some very interesting physics at work here that the authors don't completely explain. As you can see in Figure 4, only beads flowing in above a specific line will form the shell. The authors state that this is because these beads possess sufficient energy to immerse themselves in the interface, at which point they sit in an energy well and are stuck. The authors cite the work of Pawel Pieranski , who showed in 1980 that colloids can form interfacial crystals due to their asymmetric charge distribution. The authors also estimate that this energy well has a magnitude of approximately <math>10^7</math> <math>k_BT</math>.
Two interesting questions are then: 1) What is the cause of the barrier that prevents some particles from forming the shell, and 2) what exactly is the energy distribution of the incident particles that leads to such a critical line shown in Figure 4?
To answer the first question, German researchers have shown that the barrier is generated simply by electrostatics . It would seem that the particles would generally prefer either a more hydrophobic or hydrophilic medium, but that once they are at the interface, they can sit in an energy well if they have the correct orientation.
In answer to the second question, Stone and coworkers say in reference to Figure 4 that "particles on flow lines below the thick solid line undergo smaller changes in trajectory and do not posses sufficient energy to overcome barriers to adsorption." I don't quite see how a change in trajectory would lend itself to changes in energy, and I find the authors' explanation somewhat unsatisfactory. An alternative explanation would be that particles above the line spend more time in contact with the core solution due simply to the geometry of the setup.
However, the authors point out that particles must be flowed in above a critical velocity before they start forming shells. Perhaps this simply indicates that both a critical energy and time are required for proper shell integration.