Investigating the use of directed magnetic assembly to create tunable colloidal fractal aggregates and DNA-linked bead-spring chains
Byrom, Julie Elizabeth
Biswal, Sibani L
Doctor of Philosophy
The purpose of this work is to develop techniques for building complex colloidal assemblies using an applied magnetic field. The greatest challenge in this field is creating sophisticated, dynamic structures from relatively simple building blocks and interactions. Particles with magnetic properties assemble via dipolar interactions into chains along the direction of the field. Additionally, it is possible to assemble nonmagnetic particles in a magnetic field if they are immersed in a magnetic fluid. This effect is known as negative magnetophoresis (the nonmagnetic particles develop a dipole in the direction antiparallel to the external field)—and the particles are effectively diamagnetic. However, the anisotropic dipolar interaction can be a limiting factor in producing complex structures, as many magnetic assemblies are one-dimensional. In this proposal, we combine paramagnetic and diamagnetic particles to create assemblies in two-dimensions (both parallel and perpendicular to the field) and demonstrate the ability to change the morphology of these assemblies simply by changing the magnetic susceptibility of the ferrofluid as well as the overall concentration and ratio of colloids in solution. These ramified aggregates may be used in the future to build gel-like networks, which can be used as novel magnetorheological fluids and will offer valuable insight into the process of gelation via dipolar interactions. Although they may lack the complexity of the fractal aggregates just described, chains of colloids are still an important analogue for studying polymer and biofilament behavior. One characteristic which can greatly affect the properties of these filaments is their flexibility. Previous attempts to create linked particle chains have been limited to persistence lengths in the rigid and semiflexible regimes, but here we describe the method we have devised to create chains that fall in the rigid, semiflexible, and flexible regimes. This involves linking the beads of the chain with long strands of DNA, of sizes varying up to the same order of the beads themselves. This creates a physical analogy of the “bead-spring” system proposed by Rouse and Zimm. We show that we can control the flexibility of the chains by either altering the length of the DNA to tune its spring constant, or alternatively we can also tune the magnetic field strength used to assemble the chains. The field strength controls the strength of the dipolar interactions between particles and regulates the interparticle spacing between beads. This can lead to greater or fewer numbers of DNA strands which are able to form bridges, and this changes the effective spring constant holding the beads together. We demonstrate that the flexibility changes predictably within a certain range of DNA sizes, and that when the DNA becomes of similar size to the particles the DNA becomes excluded from the chain and the chains subsequently become more rigid. Unlike chains linked with smaller molecules, these chains are very resilient to shear and torque, which will allow them to be used to study filament buckling and the development of bending instabilities in complex magnetic fields or flow patterns. Finally, we explore the use of shorter DNA linkers to create batches of chains which can exhibit different flexibilities at different temperatures. This is done by exploiting the melting behavior of DNA, because micron sized particles do not exhibit the sharp melting transitions of DNA-linked nanoparticles due to the lower density of DNA on the surface and their inability to melt cooperatively. Thus, as portions of the DNA linkers are melted we see a gradual increase in flexibility of the chains with a minimal amount of breakage along the chain. This process should be reversible and would allow us to create more versatile solutions of chains. Additionally, since the DNA on larger particles has a broader melting curve, we can use them to study the effects of the initial unbinding events, which would not be possible in a system where cooperative melting cascades create sharp melting transitions. Overall, this thesis provides novel insights into the use of directed magnetic assembly to create complex colloidal structures.
Colloids; DNA; directed assembly; fractals; polymer physics