Alexander L. Yarin

Bending instability of electrically charged liquid jets of polymer solutions in electrospinning

Nanofibers of polymers were electrospun by creating an electrically charged jet of polymer solution at a pendent droplet. After the jet flowed away from the droplet in a nearly straight line, it bent into a complex path and other changes in shape occurred, during which electrical forces stretched and thinned it by very large ratios. After the solvent evaporated, birefringent nanofibers were left. In this article the reasons for the instability are analyzed and explained using a mathematical model. The rheological complexity of the polymer solution is included, which allows consideration of viscoelastic jets. It is shown that the longitudinal stress caused by the external electric field acting on the charge carried by the jet stabilized the straight jet for some distance. Then a lateral perturbation grew in response to the repulsive forces between adjacent elements of charge carried by the jet. The motion of segments of the jet grew rapidly into an electrically driven bending instability. The three-dimensional paths of continuous jets were calculated, both in the nearly straight region where the instability grew slowly and in the region where the bending dominated the path of the jet. The mathematical model provides a reasonable representation of the experimental data, particularly of the jet paths determined from high speed videographic observations.

Springer handbook of experimental fluid mechanics

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Bending instability in electrospinning of nanofibers

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Electrostatic field-assisted alignment of electrospun nanofibres

A localized approximation was developed to calculate the bending electric force acting on an electrified polymer jet, which is a key element of the electrospinning process for manufacturing of nanofibers. Using this force, a far reaching analogy between the electrically driven bending instability and the aerodynamically driven instability was established. Continuous, quasi-one-dimensional, partial differential equations were derived and used to predict the growth rate of small electrically driven bending perturbations of a liquid column. A discretized form of these equations, that accounts for solvent evaporation and polymer solidification, was used to calculate the jet paths during the course of nonlinear bending instability leading to formation of large loops and resulting in nanofibers. The results of the calculations are compared to the experimental data acquired in the present work. Agreement of theory and experiment is discussed.

Taylor cone and jetting from liquid droplets in electrospinning of nanofibers

This paper describes an electrostatic field-assisted assembly technique combined with an electrospinning process used to position and align individual nanofibres (NFs) on a tapered and grounded wheel-like bobbin. The bobbin is able to wind a continuously as-spun nanofibre at its tip-like edge. The alignment approach has resulted in polyethylene oxide-based NFs with diameters ranging from 100-300 nm and lengths of up to hundreds of microns. The results demonstrate the effectiveness of this new approach for assembling NFs in parallel arrays while being able to control the average separation between the fibres.

Impact of drops on solid surfaces: self-similar capillary waves, and splashing as a new type of kinematic discontinuity

Sessile and pendant droplets of polymer solutions acquire stable shapes when they are electrically charged by applying an electrical potential difference between the droplet and a flat plate, if the potential is not too large. These stable shapes result only from equilibrium of the electric forces and surface tension in the cases of inviscid, Newtonian, and viscoelastic liquids. In liquids with a nonrelaxing elastic force, that force also affects the shapes. It is widely assumed that when the critical potential φ0* has been reached and any further increase will destroy the equilibrium, the liquid body acquires a conical shape referred to as the Taylor cone, having a half angle of 49.3°. In the present work we show that the Taylor cone corresponds essentially to a specific self-similar solution, whereas there exist nonself-similar solutions which do not tend toward a Taylor cone. Thus, the Taylor cone does not represent a unique critical shape: there exists another shape, which is not self-similar. The exp...

Formation of nanofiber crossbars in electrospinning

The impact of drops impinging one by one on a solid surface is studied experimentally and theoretically. The impact process is observed by means of a charge-coupled-device camera, its pictures processed by computer. Low-velocity impact results in spreading and in propagation of capillary waves, whereas at higher velocities splashing (i.e. the emergence of a cloud of small secondary droplets, absent in the former case) sets in. Capillary waves are studied in some detail in separate experiments. The dynamics of the extension of liquid lamellae produced by an impact in the case of splashing is recorded. The secondary-droplet size distributions and the total volume of these droplets are measured, and the splashing threshold is found as a function of the impact parameters. The pattern of the capillary waves is predicted to be self-similar. The calculated wave profile agrees well with the experimental data. It is shown theoretically that the splashing threshold corresponds to the onset of a velocity discontinuity propagating over the liquid layer on the wall. This discontinuity shows several aspects of a shock. In an incompressible liquid such a discontinuity can only exist in the presence of a sink at its front. The latter results in the emergence of a circular crown-like sheet virtually normal to the wall and propagating with the discontinuity. It is predicted theoretically and recorded in the experiment. The crown is unstable owing to the formation of cusps at the free rim at its top edge, which results in the splashing effect. The onset velocity of splashing and the rate of propagation of the kinematic discontinuity are calculated and the theoretical results agree fairly well with the experimental data. The structure of the discontinuity is shown to match the outer solution.

Nanofiber garlands of polycaprolactone by electrospinning

In this letter, we report on a technique for the hierarchical assembly of nanofibers into crossbar nanostructures. An electrospinning process is used to create polymer-based nanofibers with diameters ranging from 10–180 nm and lengths of up to several centimeters. By controlling the electrostatic field and the polymer rheology, the nanofibers can be assembled into parallel periodic arrays. We also propose a theoretical model for the process.

Chondrogenic differentiation of human mesenchymal stem cells on oriented nanofibrous scaffolds: engineering the superficial zone of articular cartilage.

Abstract Over a period of time, the typical path of a single jet of polymer solution, in the electrospinning process follows the nearly straight electric field lines for a certain distance away from the tip, and then develops a series of electrically driven bending instabilities that cause the path of the jet to explore a cone shaped envelope as the jet elongates and dries into a nanofiber. The multitudes of open loops that are formed are rarely observed to come into contact with each other until the dry nanofiber is collected at the end of the process. A new phenomenon is reported in this paper. Electrospinning a solution of polycaprolactone in acetone caused the dramatic appearance of a fluffy, columnar network of fibers that moved slowly in large loops and long curves. The name ‘garland’ was given to the columnar network. Open loops of the single jet came into contact just after the onset of the bending instability and then merged into a cross-linked network that created and maintained the garland. Contacts between loops occurred when the plane of some of the leading loops of the jet rotated around a radius of the loop. Then a small following loop, expanding in a different plane, intersected a leading loop that was as many as several turns ahead. Mechanical forces overcame the repulsive forces from the charge carried by the jet, the open loops in flight made contact and merged at the contact point, to form closed loops. The closed loops constrained the motion to form a fluffy network that stretched and became a long roughly cylindrical column a few millimeters in diameter. This garland, which was electrically charged, developed a path of large open loops that are characteristic of a large-scale electrically driven bending instability. Over a long period of time, the fluffy garland never traveled outside a conical envelope similar to, but larger than the conical envelope associated with the bending instability of a single jet.

Evaporation of acoustically levitated droplets

Cell differentiation, adhesion, and orientation are known to influence the functionality of both natural and engineered tissues, such as articular cartilage. Several attempts have been devised to regulate these important cellular behaviors, including application of inexpensive but efficient electrospinning that can produce patterned extracellular matrix (ECM) features. Electrospun and oriented polycaprolactone (PCL) scaffolds (500 or 3000 nm fiber diameter) were created, and human mesenchymal stem cells (hMSCs) were cultured on these scaffolds. Cell viability, morphology, and orientation on the fibrous scaffolds were quantitatively determined as a function of time. While the fiber-guided initial cell orientation was maintained even after 5 weeks, cells cultured in the chondrogenic media proliferated and differentiated into the chondrogenic lineage, suggesting that cell orientation is controlled by the physical cues and minimally influenced by the soluble factors. Based on assessment by the chondrogenic markers, use of the nanofibrous scaffold (500 nm) appears to enhance the chondrogenic differentiation. These findings indicate that hMSCs seeded on a controllable PCL scaffold may lead to an alternate methodology to mimic the cell and ECM organization that is found, for example, in the superficial zone of articular cartilage.