A pinboard by
Steve Kuei

I am a Ph.D student at Rice University, working with Dr. Sibani Lisa Biswal.


I study the dynamics of single fibers in solution, how their dynamics can be predicted and controlled, and the effect they have on bulk solution. As an everyday thought experiment, one can imagine a system akin to hair in a pool of water, and how strands behave when the liquid is forced, such as when it is flushed or drained; each individual strand may be inconsequential, but the cumulative effect can have considerable effects, such as clogging of a drain. There is also the reverse, where perhaps driving the motion of the strand can in turn cause the liquid to flow.

As it is difficult to study these effects at the molecular level, we work with a magnetic colloidal particle chain system which is larger and easier to control than molecular chains, but still 10-100 times smaller in scale than fibers such as human hair. On the one hand, we use these chains as a model system with which to study the forces and scaling arguments of molecular systems for many real materials, such as carbon nanotube solutions and plastics. On the other hand, we are also able to control these chains using external magnets to act as micro-machines, forming miniature pumps, mixers, and swimmers.


Directing assembly of DNA-coated colloids with magnetic fields to generate rigid, semiflexible, and flexible chains.

Abstract: We report the formation of colloidal macromolecules consisting of chains of micron-sized paramagnetic particles assembled using a magnetic field and linked with DNA. The interparticle spacing and chain flexibility were controlled by varying the magnetic field strength and the linker spring constant. Variations in the DNA lengths allowed for the generation of chains with an improved range of flexibility as compared to previous studies. These chains adopted the rigid-rod, semiflexible, and flexible conformations that are characteristic of linear polymer systems. These assembly techniques were investigated to determine the effects of the nanoscale DNA linker properties on the properties of the microscale colloidal chains. With stiff DNA linkers (564 base pairs) the chains were only stable at moderate to high field strengths and produced rigid chains. For flexible DNA linkers (8000 base pairs), high magnetic field strengths caused the linkers to be excluded from the gap between the particles, leading to a transition from very flexible chains at low field strengths to semiflexible chains at high field strengths. In the intermediate range of linker sizes, the chains exhibited predictable behavior, demonstrating increased flexibility with longer DNA linker length or smaller linking field strengths. This study provides insight into the process of directed assembly using magnetic fields and DNA by precisely tuning the components to generate colloidal analogues of linear macromolecular chains.

Pub.: 24 Jul '14, Pinned: 29 Jun '17