opaque, it shows up more clearly on dark or slick surfaces than the typical inks used in ballpoint or felt tip pens. Gel pens can be used for many types of writing and illustration. Since the gel medium eliminates the constraints of a soluble dye, many new colors are made possible, as well as some special types of ink; gel pens are available in a wide range of vibrant or saturated colors, in pastel colors, in neon colors, in metallic colors, in glitter inks, in glow-in-the-dark ink, and so on. A rollerball pen is a pen that dispenses a water-based ink through a ball tip similar to that of a ballpoint pen. As such, gel pens might be considered a subcategory of rollerball pens; however, due to the widespread knowledge and use of the term 'gel pen', 'rollerball' is in practice typically reserved for pens which use liquid ink. The lower viscosity of rollerball ink compared to oil-based ballpoint pen ink has several effects on the pen's performance. Since the ink flows more easily and is more easily absorbed into paper, more ink is dispensed in general. This changes the writing experience by lubricating the motion of the tip over the paper. It also results in a solid and uninterrupted line, since the diffusion of the ink through the paper fills small gaps that might otherwise be left by the ball point. Compared to ballpoint pens, which dispense a smaller amount of more viscous ink, the writing by a rollerball pen takes longer to dry on the page and can seep through thin paper such as to become visible on the opposite side. When the tip of a rollerball pen is held against paper, ink leaves the tip continually by capillary action in much the same way as would occur with a fountain pen. This can lead to ink blots or smears. The rollerball pen was initially designed to combine the convenience of a ballpoint pen with the smooth "wet ink" effect of a fountain pen. Refillable rollerball pens have recently become available; these generally use cartridges of fountain pen ink. A fountain pen uses water-based liquid ink delivered through a nib, which is in general a flat piece of metal with a thin slit extending inwards from the writing tip. Driven by gravity, the ink flows from a reservoir to the nib through a feed, which is in general