Computer simulations show that end-grafted immiscible homopolymers can confer multivalence to nanoparticles, resulting in soft nanopolyhedra with structures identical to those found in small clusters of colloidal microspheres. Unprecedented structure tunability is demonstrated by several structure transition sequences, including a reentrant transition, induced by varying composition, polymer lengths, or grafting patterns. These results suggest a new method for fabricating nanoparticles with precisely controlled numbers and locations of functional sites (i.e., multivalent nanoparticles).

On agar surface, bacterial daughter cells form a 4-cell array after the first two rounds of division, and this phenomenon has been previously attributed to a balancing of interactions among the daughter bacteria and the underneath agar. We studied further the organization and development of colony after additional generations. By confocal laser scanning microscopy and real-time imaging, we observed that bacterial cells were able to self-organize and resulted in a near circular micro-colony consisting of monolayer cells. After continuous dividing, bacteria transited from two-dimensional expansion into three-dimensional growth and formed two to multi-layers in the center but retained a monolayer in the outer ring of the circular colony. The transverse width of this outer ring appeared to be approximately constant once the micro-colony reached a certain age. This observation supports the notion that balanced interplays of the forces involved lead to a gross morphology as the bacteria divide into offspring on agar surface. In this case, the result is due to a balance between the expansion force of the dividing bacteria, the non-covalent force among bacterial offspring and that between bacteria and substratum.

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The interactions between nanoparticles coated by end-grafted homopolymers of comparable size are studied using Edwards' self-consistent field (SCF) theory. The equilibrium monomer density distribution is obtained by solving the SCF equations in bispherical coordinates. It is shown that, because of the spherical geometry and redistribution of monomer density, end-grafted homopolymers in good solvent environment can induce attractive interactions. This result suggests that steric stabilization of nanoparticles requires new working principles and that the equilibrium distance between nanoparticles may be tuned by end-grafted polymers.

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