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.
Abstract: The motion of both free and tethered polymer molecules as well as rigid Brownian rods in unbound shear flow is found to be characterized by a clear periodicity or tumbling frequency. Periodicity is shown using a combination of single molecule DNA experiments and computer simulations. In all cases, we develop scaling laws for this behavior and demonstrate that the frequency of characteristic periodic motion scales sublinearly with flow rate.
Pub.: 11 Aug '05, Pinned: 30 Jun '17
Abstract: A simple and fast numerical method is developed capable of accurately determining the 3D rotational dynamics of a magnetic particle chain in an infinite fluid domain. The focus is to control the alternating breakup and reformation of the bead chain which we believe is essential to achieve effective fluid mixing at small scales. The numerical scheme makes use of magnetic dipole moments and extended forms of the Oseen-Burgers tensor to account for both the magnetic and hydrodynamic interactions between the particles. It is shown that the inclusion of hydrodynamic interaction between the particles is crucial to obtain a good description of the particle dynamics. Only a small error of deviation is observed when benchmarking the numerical scheme against a more computationally intensive method, the direct simulation method. The numerical results are compared with experiments and the simulated rotational dynamics correspond well with those obtained from video-microscopy experiments qualitatively and quantitatively. In addition, a dimensionless number (R(T)) is derived as the sole control parameter for the rotational bead chain dynamics. Numerically and experimentally, R(T)≈ 1 is the boundary between rigid "rod" and dynamic "breaking and reformation" behaviors.
Pub.: 12 Dec '12, Pinned: 30 Jun '17
Abstract: This review of the properties of flexible magnetic filaments covers the problems of their buckling under the action of a magnetic field. The dependence of the buckling on the magnetic properties of the filaments is considered. It is shown that these filaments may be used as artificial analogues of structures with important roles in the living world: cilia, elastic tails of self‐propelling microorganisms etc. The main methods used to study the relevant phenomena are considered: models, numerical simulation algorithms, and the results of linear stability analysis. Special emphasis is placed on the possible uses of the flexible magnetic filaments as model systems to study the dynamics of magnetic fields using magnetometers based on optical detection of the magnetic resonance of NV− centers in diamonds.
Pub.: 25 Feb '16, Pinned: 30 Jun '17
Abstract: Microorganisms such as bacteria and many eukaryotic cells propel themselves with hair-like structures known as flagella, which can exhibit a variety of structures and movement patterns. For example, bacterial flagella are helically shaped and driven at their bases by a reversible rotary engine, which rotates the attached flagellum to give a motion similar to that of a corkscrew. In contrast, eukaryotic cells use flagella that resemble elastic rods and exhibit a beating motion: internally generated stresses give rise to a series of bends that propagate towards the tip. In contrast to this variety of swimming strategies encountered in nature, a controlled swimming motion of artificial micrometre-sized structures has not yet been realized. Here we show that a linear chain of colloidal magnetic particles linked by DNA and attached to a red blood cell can act as a flexible artificial flagellum. The filament aligns with an external uniform magnetic field and is readily actuated by oscillating a transverse field. We find that the actuation induces a beating pattern that propels the structure, and that the external fields can be adjusted to control the velocity and the direction of motion.
Pub.: 07 Oct '05, Pinned: 29 Jun '17
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
Abstract: Microbot propulsion has seen increasing interest in recent years as artificial methods that overcome the well-established reversible and challenging nature of microscale fluid mechanics. While controlled movement is an important feature of microbot action, many envisioned applications also involve cargo transport where microbots must be able to load and unload contents on command while tolerating complex solution chemistry. Here we introduce a physical method that uses flexible and linked superparamagnetic colloidal chains, which can form closed rings or "lassos" in the presence of a planar rotating magnetic field. By adding an additional AC magnetic field along the direction perpendicular to the substrate, we can orient the lasso at a tilted camber angle. We show that these magnetic lassos can roll at substantial velocities, with precise spatial control by manipulating both field strength and phase lag. Moreover, the lasso can curl around and capture cargo tightly and transport it based on a wheel-type mechanism. At the targeted destination, cargos are easily released upon field removal and the lasso can be readily re-used. Since the entire process is physically controlled with no chemistry for attachment or disengagement involved, our system can potentially be used for transporting diverse types of cargo under different solution conditions.
Pub.: 21 Mar '17, Pinned: 29 Jun '17