Ningbin Bu

Versatile, kinetically controlled, high precision electrohydrodynamic writing of micro/nanofibers

Direct writing of hierarchical micro/nanofibers have recently gained popularity in flexible/stretchable electronics due to its low cost, simple process and high throughput. A kinetically controlled mechanoelectrospinning (MES) is developed to directly write diversified hierarchical micro/nanofibers in a continuous and programmable manner. Unlike conventional near-field electrospinning, our MES method introduces a mechanical drawing force, to simultaneously enhance the positioning accuracy and morphology controllability. The MES is predominantly controlled by the substrate speed, the nozzle-to-substrate distance, and the applied voltage. As a demonstration, smooth straight, serpentine, self-similar, and bead-on-string structures are direct-written on silicon/elastomer substrates with a resolution of 200 nm. It is believed that MES can promote the low-cost, high precision fabrication of flexible/stretchable electronics or enable the direct writing of the sacrificial structures for nanoscale lithography.

Continuously Tunable and Oriented Nanofiber Direct-Written by Mechano-Electrospinning

A mechano-electrospinning method is presented to direct-write oriented nanofiber with high deposition accuracy in continuously tunable manner. This method uses a large nozzle to deposit high-resolution fiber-arrays by near-field localization. Due to the existence of mechanical drawing force, a higher resolution pattern can be direct-written by a lower voltage, which just keeps the Taylor cone stable. Then the fiber is stretched through a moving substrate, by which the deposited fiber can be continuously tuned from 400 nm to 200 nm in a linear relationship. The effects of the speed on the diameter and morphology of fibers are studied, and the analysis of the mechanism of the fiber alignment is given. The positionability and controllability make mechano-electrospinning very different from the traditional electrospinning, which only collects the fibers in the form of nonwoven fabric. In addition, the fibers fabricated by this method can directly deposit over a large flat area in the form of arrays and complex patterns with high precision.

Controllable self-organization of colloid microarrays based on finite length effects of electrospun ribbons

This paper presents a mechanoelectrospinning (MES)-assisted surface-tension driven self-organization to provide a possible route towards inexpensive generation of large-scale ordered microarrays in a controllable manner. To control the self-organization driven by surface tension and Plateau–Rayleigh instability, finite length effects are utilized to manipulate the self-organizing processes and adjust the competition between nucleation and free surface instability. We introduce fine ribbon-lattices to determine the boundary conditions of ribbons to make use of the finite length effects. The ribbon-lattices are electrodeposited precisely by MES, borrowing ideas from the “Chinese kite”, by involving the mechanical drawing force and the electric field force. Then the samples are transferred to a moisture-rich environment in which the ribbons absorb water vapour and become liquid lines. Surface instability emerges and leads the liquid lines to controllable self-organization. We uncover the controllable area to manipulate the self-organization behavior. A uniform or hierarchical microarray with a specific position, gap and droplet-size can be generated in a continuously tunable manner. This bottom-up method provides a digital approach for the fabrication of large-scale ordered microarrays and micropatterns.

Tunable bead-on-string microstructures fabricated by mechano-electrospinning

In this paper, bead-on-string microstructures are fabricated by the mechano-electrospinning (MES) process in a continuously tunable manner. The thin jet is pulled onto the substrate by the stable electric field force and tunable mechanical drawing force, and then the bead-on-string structures are generated by means of the force exerted on the jet, which changes from capillary force and resisting viscosity force to friction force at the contact point in the horizontal direction. In a stable bead-on-string formation process, one cycle can be divided into three stages from the point of view of the jet behaviour: being anchored, being stretched, and skipping. The bead size and the bead gap are continuously tunable through the MES process. The fabrication mechanisms of the bead-on-string microstructure are uncovered through theoretical analysis and experimental characterization. When a critical velocity is achieved, the jet directly falls on the substrate without accumulation since the mechanical drawing force in the horizontal direction overtakes the capillary force, which leads the bead-on-string microstructures to a continuous fibre line. It is a flexible and highly controllable method to fabricate bead-on-string microstructures.

Near-field behavior of electrified jet under moving substrate constrains

We investigate the dynamics and shapes of electrified jet deposited onto a moving substrate in near-field electrospinning. At low speed, drag effect imposes on the jet and makes it buckling to a ‘heel’. As the ‘heel’ continues to move far away, a restoring force is accumulated until it is large enough to make an ‘out of the plane deformation’, which will also introduce torsion for the jet and turns it into a rotation state. When the speed increases, stretching effect makes jet drawing to a stable catenary shape. The ‘heel’ is a transition stage between catenary and rotation state due to the buckling of the jet. Moreover, the transformation from the ‘heel’ to ‘catenary’ is validated by modeling the jet as electrified filament. The simulation results show that the speed brings the pulling force exerted on the jet tail and it only depends on the substrate speed. The works provide a better understanding the effect mechanism of the substrate speed on the fiber morphology.