Rollerball pens are pen

Pen

A pen is a device used to apply ink to a surface, usually paper, for writing or drawing. Historically, reed pens, quill pens, and dip pens were used, with a nib of some sort to be dipped in the ink. Ruling pens allow precise adjustment of line width, and still find a few specialized uses, but...

s which use ball point writing mechanisms with water-based liquid or gelled ink

Ink

Ink is a liquid or paste that contains pigments and/or dyes and is used to color a surface to produce an image, text, or design. Ink is used for drawing and/or writing with a pen, brush, or quill...

, as opposed to the oil-based viscous

Viscosity

Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

 inks found in ballpoint pen

Ballpoint pen

A ballpoint pen is a writing instrument with an internal ink reservoir and a sphere for a point. The internal chamber is filled with a viscous ink that is dispensed at its tip during use by the rolling action of a small sphere...

s. The characteristics of these less viscous inks, which tend to saturate more deeply and more widely into paper

Paper

Paper is a thin material mainly used for writing upon, printing upon, drawing or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....

 than other types of ink, give rollerball pens their distinctive writing qualities.

The rollerball pen was initially designed to combine the convenience of a ballpoint pen

Ballpoint pen

A ballpoint pen is a writing instrument with an internal ink reservoir and a sphere for a point. The internal chamber is filled with a viscous ink that is dispensed at its tip during use by the rolling action of a small sphere...

 with the smooth "wet ink" effect of a fountain pen

Fountain pen

A fountain pen is a nib pen that, unlike its predecessor the dip pen, contains an internal reservoir of water-based liquid ink. The pen draws ink from the reservoir through a feed to the nib and deposits it on paper via a combination of gravity and capillary action...

.