ELECTROSPINNING | THE PROCESS

Whipping Instabilities

Electrospinning based technique is defined as a technique that allows to obtain nanofibers, using for this purpose an electrified liquid or a molten polymer. It shares characteristics of electrospray process and other conventional spinning processes although coagulation bath and high temperatures are not required. This issue makes the electrospinning process suites to the production of fibers using polymers with long-chain molecules.

When a high voltaje HV is applied to a liquid meniscus (from a solution or a molten polymer) that pokes out from the tip of a capillary tube, the liquid becomes charged and is produced a balance of forces between electrical repulsion and the surface tension of the liquid. As a consequence, the meniscus is stretched and the spherical shape of the liquid turns into a cone, called Taylor cone in honor of G.I.Taylor, who was the first who described the conical geometry. From the tip of the cone, a thin jet is emitted. Due to varicose efforts, the jet should break up in a cloud made of infinite droplets. But if the molecular cohesion of the liquid is sufficiently high, stream breakup does not occur (if it does, droplets are electrosprayed) and a charged liquid jet is formed.

The jet is then elongated by a whipping process caused by electrostatic repulsion initiated at small bends in the fiber, until it is finally deposited on the grounded collector. The elongation and thinning of the fiber resulting from this bending instability leads to the formation of uniform fibers in the micro/nano scale.

YFLOW® ELECTROSPINNING TECH. | BACKGROUND

In the 16th century, W. Gilbert described the behavior of magnetic and electrostatic phenomena after studying the behavior of a droplet of water placed close to an electrically charged piece of amber. The spherical droplet of water turned to form a cone shape and small droplets were ejected from the tip of the cone, which resulted into the first recorded observation of electrospray. Boys (19^th century) also described the phenomenon and designed an apparatus consisting of a small and insulated dish connected to an electrical machine. He noticed that he could spin fibers from different kind of waxes. Few years after (1900), JF Cooley and WJ Morton patented the process of electrospinning. A great advance was achieved in 1914 when John Zelezny published his work about the behavior of fluid droplets at the end of metal capillaries. He also tried to develop a model the behavior of fluids under electrostatic forces. Anton Formhals was the first who tried to apply the electrospinning technique to industry. He described in a sequence of patents from 1934 to 1944 for the fabrication of textile yarns. Simultaneously, CL Norton electrospun a molten polymer using an air-blast to assist fiber formation.

GI Taylor (1964-1969) developed the theoretical underpinning of electrospinning, which contributed to electrospinning by mathematically modeling the shape of the cone formed by the fluid droplet under the effect of an electric field; this characteristic droplet shape is now known as the Taylor cone.

In the early 1990s Reneker´s and Rutledge´s investigation groups popularized the name electrospinning for the process and demonstrated that themoset and thermoplastic polymers could be electrospun into nanofibers using polymer solutions (thermosets) or molten polymers (thermoplastics). This fact made possible that the number of publications about electrospinning has been increasing exponentially every year.

In recent years there has been a high increase in such developing technologies based on nanoscale polymer fibers and has been developed many theoretical studies of the driving mechanisms of the electrospinning process (formation of cone-jet, bending instabilities, …).

YFLOW® ELECTROSPINNING TECH. | FEATURES

• – Generation and fine control of single jet with diameter sizes ranging from tens of microns to a few nanometers.
• – Full control of diameter size varying flow rate and conductivity of outer liquid.
• – Energy saving: No need for a freezing chamber (Spray Chilling) or drying (Spray Drying) or a chemical reactor to solidify the nanofibers.
• – It is ensured a low dispersion of the diameter-size of nanofibers.
• – Null coalescence risk.
• – The use of Yflow® Multi-Injection and Continuous Collection Devices ensure the Scale-Up processes for high throughput production.

YFLOW® ELECTROSPINNING TECH. | BASIC SETUP

The standard laboratory setup for electrospinning consists of a spinneret (a needle or a hole) connected to a HV-DC power supply, a syringe pump, and a grounded collector separated from the spinneret a few centimeters. Emulsions, nanoparticle suspension, polymeric or sol-gel solution or a molten polymer can be processed by electrospinning. The liquid is placed into the syringe located in the syringe pump, which pumps the liquid to the tip of the spinneret, where is extruded at a continuous flow rate pump. This condition (constant flow rate) can be replaced by constant pressure condition using a header tank (for feeding), which is highly recommended if a low viscosity liquid is devoted to be electrospinned.
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Sketch of the Setup

YFLOW® ELECTROSPINNING TECH. | SCALING UP

Yflow® Multiplexed Nozzle Technology enables to increase the throughput of the electrospinning process. Homogeneous Nanofiber sheets can be obtained with a relative displacement between Multi-Nozzle Device and the collector. The layer´s thickness can be produced from tens of nanometers up to several milimeters.