Varying any of these parameters even by small amounts can have a large effect on the structure of fibres produced; this enables the formation of fibres with defined features such as fibre diameter, flat ribbon or cylindrical fibres, level of fibre surface porosity and bead formation. Depending on the intended application these properties have the potential to be selected and specifically expressed. For example thinner fibres may be preferred due to the larger surface area that they convey but with small diameter comes an inherent reduction in fibre strength. Aligned and random fibre mats can be formed by changing the collector plate and by manipulating the external electric field. Collecting on a non flat or porous surface can be beneficial as it allows for faster drying of the deposited fibres reducing the moulding together effect which can deteriorate fibre properties. This is particularly applicable for aqueous polymer solutions due to the lower volatility in comparison to other solvents commonly associated with electrospinning. However it was noted that collection on a smooth surface may result in a more densely packed fibre structure due to the wicking and diffusion of the residual solvent molecules allowing the fibres to be pulled together while conducting away residual charges [22]. Moving collectors are often used for the fabrication of aligned fibres but can also be useful for use with a system where the solvent has a high boiling point as this increases the rate of evaporation [23]. Ribbon or flattened fibres can be formed by a mechanism relating to the solvent evaporation rate during the electrospinning. Should the polymer-solvent system be at such a state where a thin polymer skin forms on the liquid jet surface as a result of hyper solvent volatility the liquid core can succumb to the atmospheric pressure allowing the fibres to collapse in on themselves at the same time as complete solvent evaporation resulting in flattened fibre formation [24]. Koski et al, (2004) observed that higher molecular weight polymer-solvent systems of
poly(vinyl alcohol) in water produced ribbon like fibres [25].