Introduction:

Spherical shells of micron-sized colloidal particles can be formed by introducing colloidal particles to emulsion droplet templates.¹  The particles self-assemble on the surface of the droplets in order to minimize the total interfacial energy² (left image), forming colloidosomes.  These structures enable us to study interparticle interactions at fluid-fluid interfaces.  The particles order into 2D spherical crystals (center image) which may yield the answer to J. J. Thomson's 97-year-old problem.³  We are also interested in using colloidosomes as encapsulation structures which exhibit selective permeability (right image).

References:
[1]    O. D. Velev, K. Furusawa, and K. Nagayama, Langmuir 12, 2374 (1996).
[2]    P. Pieranski, Phys. Rev. Lett. 45, 569 (1980).
[3]    J. J. Thomson, Philos. Mag. 7, 237 (1904).
 
 

 

  Interparticle Interactions:

Particles self-assemble on an oil-water interface to minimize total interfacial energy.  On a spherical interface, we have observed a secondary minimum in the interparticle potential (in addition to the primary van der Waals minimum).  Since the particles exhibit long-range dipole-dipole repulsion, there must be an additional attractive interaction causing this minimum.  We are currently investigating this attraction.

  Particle Ordering:

In 1904, J. J. Thomson posed the problem of finding the minimal Coulomb energy configuration of N unit point charges distributed on the two-dimensional surface of a conducting sphere.  Nonzero curvature necessitates defects in the 2D crystalline structure formed by the particles.  Originally introduced as a model for the structure of atoms, it is also an interesting optimization problem in computational physics.  We are working to solve this problem with Mark Bowick (Syracuse), David Nelson (Harvard), and co-workers, whose use of continuum elastic theory in numerical simulations has predicted isolated disclinations screened by terminating dislocation branches.

Reference:  M. J. Bowick, D. R. Nelson, and A. Travesset, Phys. Rev. B 62, 8738 (2000).

  Encapsulation:

The encapsulation of materials is an area of active research with endless application in industry and medicine.  For example, there is strong interest in encapsulation and controlled release of materials ranging from fragrances and flavors in foods to molecules produced by sensitive biomaterials such as living cells.  We have made colloidosomes which are selectively permeable to particles of different sizes, making them appropriate for controlled release applications.  We are working with Manuel Marquez-Sanchez (Kraft) to explore more potential uses of colloidosomes.

The mechanical deformation and rupture of colloidosome are also of interest in possible encapsulation applications as well as being of intrinsic interest. Controlled radial force is applied to the sintered and polyelectrolyte-stabilized colloidosomes using a calibrated glass microcantilever, allowing the resulting deformation and breakage to be characterized.  Finite Element Modeling (FEM) has been applied to such deformation, in cooperation with Xi Chen and John Hutchinson, allowing a better determination and understanding of dominant colloidosome characteristics. 

                       

Microcantilever indents sintered colloidosome (optical microscope).                Sintered colloidosomes indented (left) and broken (right) (Scanning Electron Microscope).

 

 

Polyelectrolyte-stabilized colloidosomes show remarkable elastic resilience to deformation.

 

For colloidosomes stabilized with poly-L-lysine, Finite Element Modeling has allowed a careful determination of these capsules’ structural characteristics.  This, in turn, has guided us to exploit the poly-L-lysine to achieve non-mechanical release triggers.  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The success of finite element modeling in describing colloidosomes stabilized with poly-L-lysine is surprising if polystyrene beads dominate the material properties of the colloidosome membrane: beads are only two orders of magnitude smaller than colloidosomes, as illustrated in Figure 5, yet our model treats the membrane as a continuum on the scale of the mesh elements.  The model’s success is unsurprising, however, if membrane properties and behavior are dominated by adsorbed poly-L-lysine, which should form a much more homogeneous coating than the bead layer.  Furthermore, we see stable colloidosomes whose surfaces are covered very sparsely with beads, leaving large inter-bead gaps, as for the lower colloidosome in Figure 5.  These observations strongly indicate that poly-L-lysine, cross-linked by absorption to beads, governs colloidosome membrane properties; if this is so, poly-L-lysine dominates these structures’ material elastic modulus, E, as well as their tensile stress, S, and these colloidosomes can be considered as poly-L-lysine capsules with the membrane, which would otherwise be soluble in water, stabilized by adsorption to crosslinking beads.

 

Of further interest to possible encapsulation applications, gold and nickel nanorods 2-15 microns in length have been incorporated into sintered and polyelectrolyte-stabilized colloidosomes, introducing the possibility of manipulating and influencing these structures through electromagnetic means as well as through selective functionalization through thiol adsorption.  This work, as well as the microcantilever work, is supported through a grant from Unilever.

Members of our group involved in this work are: Ming Hsu, Vernita Gordon, Anna Pilipienko, and Onyinye Ofoegbu. In the past, Andreas Bausch, Michael Nikolaides, and Tony Dinsmore were also involved.

Page maintained by:
Ming Hsu
9 & 15 Oxford Street, McKay Laboratory
Department of Physics
Harvard University
Cambridge, MA 02138
617-496-3978
mailto:mvalenti@deas.harvard.edu