Civil Engineering Reference
In-Depth Information
electrospraying process [14, 15]. Electrospraying is a very old technique which was i rst
observed in 1897 by Rayleigh and studied in detail by Zeleny in 1914 [16]. h e i rst pat-
ent on electrospinning was i led in 1934 by Formhals [17]. Although the term "electro-
spinning," derived from "electrostatic spinning," was used relatively recently (in around
1994), its fundamental idea dates back more than 60 years earlier. From 1934 to 1944,
Formhals published a series of patents [17-21] describing an experimental setup for
the production of polymer i laments using an electrostatic force. A polymer solution,
such as cellulose acetate, was introduced into the electric i eld. h e polymer i laments
were formed, from the solution, between two electrodes bearing electrical charges of
opposite polarity. One of the electrodes was placed into the solution and the other onto
a collector. Once the solution ejected out of a metal spinneret with a small hole, the
charged solution jets evaporated to become i bers which were collected on the collec-
tor. h e potential dif erence depended on the properties of the spinning solution, such
as polymer molecular weight and viscosity. When the distance between the spinneret
and the collecting device was short, spun i bers tended to stick to the collecting device
as well as to each other, due to incomplete solvent evaporation. Formhals [19] spinning
setup consisted of a movable thread collecting device in the form of a spinning drum
similar to the one in conventional spinning, on which the threads were collected in a
stretched state. However, the setup was capable of producing parallel threads onto the
collection device in such a fashion that they can be unwound continuously. h e main
disadvantage of the spinning method designed by Formhals [18, 19] was that it was dif-
i cult to completely dry the i bers due to the short distance between the spinning zone
and the collector. In 1939 he revised the disadvantages of the earlier setup by altering
the distance between the spinneret and the collection device to get more drying time
for the electrospun i bers, and patented the method. In 1952, Vonnegut and Neubauer
were able to produce streams of highly-electrii ed uniform droplets of about 0.1 mm in
diameter [22] . h ey invented a simple apparatus for the electrical atomization. A glass
tube was drawn down to a capillary having a diameter in the order of a few tenths of
a millimeter. h e tube was i lled with water or some other liquid and an electric wire
connected with a source of variable high voltage (5-10 kV) was introduced into the liq-
uid. In 1955, Drozin [23] investigated the dispersion of a series of liquids into aerosols
under high electric potentials. A glass tube ending in a i ne capillary similar to the one
employed by Vonnegut and Neubauer was used in the study. h e study found that for
certain liquids and under proper conditions, the liquid was issued from the capillary as
a highly dispersed aerosol consisting of droplets with a relatively uniform size. In 1966,
Simons [24] patented an apparatus for the production of nonwoven fabrics of ultra-thin
and very light weight with dif erent patterns using electrical spinning. h e positive elec-
trode was immersed into the polymer solution and the negative one was connected to a
belt where the nonwoven fabric was collected. It was found that i bers from low viscos-
ity solutions tended to be shorter and i ner, whereas those from more viscous solutions
were relatively continuous. In the 1960s, Tylor initiated fundamental studies on the
jet forming process in electrospinning. Tylor studied the shape of the polymer droplet
produced at the tip of the needle when an electric i eld is applied and showed that as
the intensity of the electric i eld is increased, the hemispherical surface of the l uid
at the tip of the capillary tube elongates to form a conical shape known as the Taylor
cone [25] . Further increasing the electric i eld, a critical value is attained with which
