About this page

This is a brief treatise on art mobiles. Its purpose is to gather observations about mobiles in one place and connect related concepts. It also enumerates several variants of the form.

The next page examines motorized and solar-powered variants in more detail.

Definition of a mobile

See the wiki definition, especially for the origin of the word.

A mobile is an artwork whose value is purely aesthetic — it exists to be seen.

A mobile is a three-dimensional sculpture.

A mobile is kinetic: it moves.

Disambiguation

The word “mobile” more generally means able to move or be moved between locations.

The word mobile has also come to mean a mobile phone or cell phone.

Characteristics of mobiles

Levels

A mobile has levels.

Each level is articulated relative to the others. Each level moves relative to the others. That movement may be rotation, or bobbing up and down.

Pivots between levels articulate the structure. They may allow either infinite rotation or constrained rotation. A swivel or bearing allows infinite rotation. A thread or chain link constrains rotation and stores torsional energy.

A mobile with only one level is still a mobile, but it might more properly be called a spinner or a bobber.

Suspension or support

Most mobiles are suspended.

Kinetic art can also be mounted from below rather than suspended. Such artworks share the same characteristics as mobiles.

George Rickey created many supported, outdoor, kinetic sculptures.

The word stabile refers to a class of Alexander Calder artworks that are sculpture but do not physically move.

Units and structure

A mobile is suspended from a pivot at its top level. The top level is one unit.

A mobile can have many recursive, pivoted units in lower levels. In other words, the mobile can be an upside-down branching tree.

Or a mobile can have just one pivot and suspended unit per level. Then the mobile is a sequence or chain.

Offsets

Many mobiles have lower units suspended offset from the pivot of the the level above. Then said offset lower unit is balanced by either a simple weight or another unit, offset to the other side of the pivot of the level above. A simple weight can be said to be a terminus in the tree structure.

When lower levels are not offset horizontally from the pivot of the level above, the mobile necessarily has a sequence structure, like a chain. Each level can still have rotating beams and the beams can have asymmetric mass and decoration, to appear similar to mobiles that branch. Each arm of the beams can be said to be a terminus.

Balance and Mass

A mobile is physically balanced. Each level balances around its pivot. Several forces are balanced: gravity and driving forces.

Each level carries the mass of all levels below it. The mass of a mobile is the mass of its top level plus the mass of all the levels below it. The levels are progressively less mass as you progress downwards, for a suspended mobile. The components may also be smaller and have less strength as you progress downwards. Conversely, the components of a mobile are usually progressively stronger and more massive as you progress upwards.

However, this is not always the case. Some mobiles have all units identical, and all components the same strength. In this case, some components are overbuilt, that is, stronger than they need to be.

Fractal nature of mobiles

Many mobiles are fractal: self-similar at different scales. The lower levels are smaller than but look similar to large upper levels. In other words, a substructure is smaller than but similar in appearance to the structure as a whole.

Symmetry

Many mobiles are asymmetric. Usually the asymmetry is in an offset, terminal mass counterbalancing a recursive substructure offset to the other side.

Even when a mobile is symmetric, it can move like other mobiles, since driving forces such as the wind may be asymmetric. But in a uniform wind, a symmetric mobile would not move much.

Helix mobiles

A helix has another kind of symmetry. A helix can be a simple helix, a spiral helix, or a double helix.

A helix suspended by a pivot to its center is also an art mobile, since it is a sculpture, is kinetic, has a pivot, and can be rotated by wind.

Visual balance

A mobile may also be visually balanced by effects of human visual perception. For example, one side of a mobile may appear the same size as another side. Or one side of a mobile may have color or texture that balances or contrasts another side.

Visual balance can be ephemeral. Physical motion may change the visual balance while maintaining physical balance.

Perceptual motion

A mobile can display perceptual motion by moire patterns at the crossing of two moving arms of a mobile.

Driving force

Traditional mobiles move by wind. Even indoors, drafts from heating, air conditioning, thermal currents, or people moving will drive them.

Some mobiles are driven by electromotive force, that is, motors.

A Crookes radiometer is a device driven by light through differential heating of its vanes in a partial vacuum. Since it is a toy and art to be seen, with a pivot, and kinetic, it is a mobile.

A Crookes radiometer has one level. It might be possible to build a Crookes radiometer with many levels.

Forces on wind driven mobiles

We use the word “vane” for an element of a mobile that has an area that catches the wind and that is offset from the pivot of the element’s level. A vane has drag in the air.

A concentrated wind on a vane spins that level. Due to the small friction of the pivot, forces also spin the level above, but to a lesser degree.

Apparent wind

When a first level spins, it moves all the other levels suspended from that level. The other levels see an apparent wind. The apparent wind is tangent to the circle circumscribed by the rotating pivot of their suspension to the first level. When the other levels have vanes, they will weather cock. That is, the other levels will spin so that their vanes are aligned with the apparent wind.

Effects of constrained pivots

Consider the case when a mobile’s pivots do not allow infinite, nearly frictionless rotation, but store torsional energy. Then a momentary wind perturbs a mobile and stores torsional energy in pivots. When the wind ceases, the torsional energy is released and the levels swing back to the resting position of their pivots. During the swing back, the vanes again see an apparent wind and weathercock.

Mobiles in a uniform wind field

Suppose a mobile is in a uniform wind field, such as in a wind tunnel with smooth, aligned, laminar flow, without turbulence. Then eventually the mobile would assume some steady state position and cease to move. This is an ideal that assumes there is no turbulent flow around the vanes of the mobile.

Mobiles in different fluids and environments

Most mobiles work in air and gravity. They are driven by air currents. The fluid is air.

Zero gravity

You can have a mobile in zero gravity. The pivots must be rigid, otherwise the mobile would likely get tangled.

Vacuum

If you consider a radiometer to be a mobile, then a mobile can work in a vacuum.

Lighter than air

You can make a mobile from lighter than air components. A mobile made from helium balloons, in a structure that is right side up tree, is upside down compared to a usual mobile.

Water

Similar to a mobile made from helium balloons, you can make a mobile from buoyant elements in water. It would not move much in small containers of stagnant water, unless there were fish in the water.