A gel pen uses ink in which pigment

Pigment

A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...

 is suspended

Suspension (chemistry)

In chemistry, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer. The internal phase is dispersed throughout the external phase through mechanical agitation, with the use of certain...

 in a water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

-based gel

Gel

A gel is a solid, jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state...

. Because the ink is thick and opaque

Opacity (optics)

Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...

, it shows up more clearly on dark or slick surfaces than the typical inks used in ballpoint

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...

 or felt tip

Marker pen

thumb|MarkerA marker pen, marking pen, felt-tip pen, flow or marker, is a pen which has its own ink-source, and usually a tip made of a porous, pressed fibres; such as felt or nylon.-Permanent marker:...

 pens. They can be used for many types of writing and illustration.

The general design of a gel pen is similar to that of a regular rollerball pen

Rollerball pen

Rollerball pens are pens which use ball point writing mechanisms with water-based liquid or gelled ink, as opposed to the oil-based viscous inks found in ballpoint pens...

, with a barrel containing the writing mechanism and a cap, and a reservoir filled with ink. The barrels can be created in many different sizes and designs; some have finger grips of rubber or plastic. The size of the nib

Nib (pen)

A nib is the part of a quill, dip pen or fountain pen which comes into contact with the writing surface in order to deposit ink. Different types of nibs vary in their purpose, shape and size, as well as the material they are made from.-Quill:...

 or pen tip ranges from 0.18 mm to 1.5 mm.

Inks

The main advantage of gel ink is its high viscosity

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...

, which supports a higher proportion of pigments in the medium. The pigments are typically copper phthalocyanine and iron oxide

Iron oxide

Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...

s, and the gel is made up of water and biopolymer

Biopolymer

Biopolymers are polymers produced by living organisms. Since they are polymers, Biopolymers contain monomeric units that are covalently bonded to form larger structures. There are three main classes of biopolymers based on the differing monomeric units used and the structure of the biopolymer formed...

s, such as xanthan gum

Xanthan gum

Xanthan gum is a polysaccharide, derived from the bacterial coat of Xanthomonas campestris, used as a food additive and rheology modifier, commonly used as a food thickening agent and a stabilizer...

 and tragacanth gum, as well as some types of polyacrylate thickeners. The pigments are opaque

Opacity (optics)

Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...

, and gel pens are available in a rainbow of bright and pastel colours, as well as opalescent

Opalescence

Opalescence is a type of dichroism seen in highly dispersed systems with little opacity. The material appears yellowish-red in transmitted light and blue in the scattered light perpendicular to the transmitted light. The phenomenon is named after the appearance of opals.There are different degrees...

, metallic, and glitter

Glitter

Glitter describes an assortment of very small pieces of copolymer plastics, aluminum foil, titanium dioxide, iron oxides, bismuth oxychloride or other materials painted in metallic, neon and iridescent colors to reflect light in a sparkling spectrum...

y colours which show up clearly on dark paper.

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 use a paste ink based on a dye

Dye

A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....

 solute in an alcohol

Alcohol

In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

 solvent

Solvent

A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...

. Compared to ballpoint pens, gel pens:

• Create a bolder line
• Smudge more, due to ink being wetter
• Have occasional skips, due to the ball being less evenly coated with ink
• Use more ink, leading to more frequent refills or replacements
• Are more likely to leak ink when the stopper fluid drains out
• May be able to write on smooth, non-absorbent materials

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...

s and liquid-ink rollerball pen

Rollerball pen

Rollerball pens are pens which use ball point writing mechanisms with water-based liquid or gelled ink, as opposed to the oil-based viscous inks found in ballpoint pens...

s use a dye solute in a water-based solvent

Solvent

A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...

. Compared to these pens, gel pens:

• Are less prone to bleed through to the other side of a piece of paper
• Create finer, more controlled lines with fewer blots
• Can create white or brightly coloured opaque lines on dark surfaces
• May be able to write on smooth, non-absorbent materials

Paint marker

Paint marker

A paint marker is a type of marker pen that is used to create permanent writing on a variety of surfaces from paper to metal to stone.-Ink:Unlike with most permanent markers the ink is an oil-based paint and generally requires shaking before use, similar to an aerosol spray paint can...

s use an oil-based paint/ink that is very opaque and overlays all other colours beneath it. Compared to paint markers, gel pens:

• Are much smaller, with finer tips
• Have no odour and do not require shaking to mix pigment with solvent

India ink

India ink

India ink is a simple black ink once widely used for writing and printing and now more commonly used for drawing, especially when inking comic books and comic strips.-Composition:...

 (based on carbon black

Soot

Soot is a general term that refers to impure carbon particles resulting from the incomplete combustion of a hydrocarbon. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolyzed fuel particles such as cenospheres,...

 pigment) is perhaps the ink closest to gel ink in opacity, although it is usually made with an alcohol and shellac

Shellac

Shellac is a resin secreted by the female lac bug, on trees in the forests of India and Thailand. It is processed and sold as dry flakes , which are dissolved in ethyl alcohol to make liquid shellac, which is used as a brush-on colorant, food glaze and wood finish...

-based binder.

Another trait of gel ink is that it resists common laboratory analysis

Forensic chemistry

Forensic chemistry is the application of chemistry to law enforcement or the failure of products or processes. Many different analytical methods may be used to reveal what chemical changes occurred during an incident, and so help reconstruct the sequence of events...

. The United States Secret Service

United States Secret Service

The United States Secret Service is a United States federal law enforcement agency that is part of the United States Department of Homeland Security. The sworn members are divided among the Special Agents and the Uniformed Division. Until March 1, 2003, the Service was part of the United States...

 has maintained the International Ink Library for many decades. Because manufacturers change their ink formulas slightly from year to year, thin-layer chromatography (TLC) can be used on ink from traditional pens to trace the manufacturer and date of manufacture of most inks. The pigments in gel ink do not dissolve, and therefore cannot be analyzed with TLC.