Materials composed of stacked plates are stronger than the same materials in bulk. However, such stacked plate materials are generally composed of hard, inorganic materials cemented together by a more flexible substance, and do not occur in long fiber shapes. Very few natural or manmade materials are composed of stacks of plate-like fibers. Manmade polymer fibers are nearly always round because they are fabricated using two-phase systems; for example, a solubilized polymer precursor or a monomer–initiator solution is introduced into air by electrospinning, extrusion, or pulling.
Hydrodynamic focusing using microfluidic systems has been reported for making solid or hollow round fibers with micron and sub-micron diameters. The fibers and tubes have been polymerized using both chemical initiators and ultraviolet (UV) polymerization. Hydrodynamic focusing has also been used to precipitate fibers in microfluidic channels as two reactive streams intersect. These fibers are roughly rectangular, as a result of the shape of the interface between the two streams. Hydrodynamic focusing can be performed using grooves in the walls of microfluidic channels to focus one stream with another to create cross-sectional stream shapes more complex than simply circular or rectangular.
In an initial demonstration of production of polymer fibers with pre-designed cross-sectional shapes, acrylate fibers were cast using a hydrodynamic focusing device with grooves in the top and bottom of a microfluidic channel that focused a miscible sheath fluid around the solubilized polymer core solution. Laminar flow minimized mixing of the sheath and core fluids, and hydrodynamic focusing of the sheath fluid controlled the cross-sectional shape of the core. The cross-sectional size was determined by the flow rate ratio of the core and sheath streams.
The fiber hardened as the solvent diffused out of the core into the sheath fluid after the hydrodynamic shaping process and during subsequent flow through the microfluidic channel. However, the range of fiber diameters that could be produced using this casting process was limited. The larger fibers tended to harden first on the outside, leading to collapse of the fibers as the internal regions shrunk during casting. The smallest diameter that could be achieved (~300 nm) was limited by the ability of the pumps to transport the viscous fluids without pulsing. The slow rate of hardening also meant that the fibers tended to become rounder after exiting the microfluidic channel.
To overcome these limitations, UV polymerization was explored as a faster, more uniform method of polymerizing fibers during the hydrodynamic shaping process. The initial challenge was to determine conditions under which the UV polymerization could be accomplished fast enough to lock in the cross-sectional shape of the fiber before it exited the microfluidic channel. The roles of flow velocity, flow-rate ratio, and UV power in creating fibers with rectangular cross-sectional shapes were explored. The microfluidic sheath flow system successfully shaped an acrylate mixture into flat fibers using hydrodynamic focusing and UV polymerization. Initial measurements of the structural and mechanical properties demonstrated shape control and fiber integrity.
Hydrodynamic focusing using passive wall structures was used to shape a prepolymer stream, which was subsequently polymerized using UV exposure. The shape designed using flow simulations was maintained, and the size of the fibers was controlled using the ratio of the flow rates of the sheath and the prepolymer. The fibers exhibited reproducible shapes over meter lengths. The fidelity of the shape was a function of both exposure time and phase matching of the sheath and prepolymer fluids. This microfluidic approach for production of fibers with defined cross-sectional shape can produce fibers for further development of materials with new or improved performance characteristics.
This work was done by Abel L. Thangawng, Peter B. Howell Jr., Christopher M. Spillmann, Jawad Naciri, and Frances S. Ligler of the Naval Research Laboratory. NRL-0050
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UV polymerization of hydrodynamically shaped fibers
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