Nanotechnology is based on the concept of tiny, self-replicating robots. The Utility Fog is a very simple extension of the idea: Suppose, instead of building the object you want atom by atom, the tiny robots linked their arms together to form a solid mass in the shape of the object you wanted? Then, when you got tired of that avant-garde coffeetable, the robots could simply shift around a little and you'd have an elegant Queen Anne piece instead.
The color and reflectivity of an object are results of its properties as an antenna in the micron wavelength region. Each robot could have an "antenna arm" that it could manipulate to vary those properties, and thus the surface of a Utility Fog object could look just about however you wanted it to. A "thin film" of robots could act as a video screen, varying their optical properties in real time.
Rather than paint the walls, coat them with Utility Fog and they can be a different color every day, or act as a floor-to-ceiling TV. Indeed, make the entire wall of the Fog and you can change the floor plan of your house to suit the occasion. Make the floor of it and never gets dirty, looks like hardwood but feels like foam rubber, and extrudes furniture in any form you desire. Indeed, your whole domestic environment can be constructed from Utility Fog; it can form any object you want (except food) and whenever you don't want an object any more, the robots that formed it spread out and form part of the floor again.
You may as well make your car of Utility Fog, too; then you can have a "new" one every day. But better than that, the *interior* of the car is filled with robots as well as its shell. You'll need to wear holographic "eyephones" to see, but the Fog will hold them up in front of your eyes and they'll feel and look as if they weren't there. Although heavier than air, the Fog is programmed to simulate its physical properties, so you can't feel it: when you move your arm, it flows out of the way. Except when there's a crash! Then it forms an instant form-fitting "seatbelt" protecting every inch of your body. You can take a 100-mph impact without messing your hair.
But you'll never have a 100-mph impact, or any other kind. Remember that each of these robots contains a fair-sized computer. They already have to be able to talk to each other and coordinate actions in a quite sophisticated way (even the original nano-assemblers have to, to build any macroscopic object). You can simply cover the road with a thick layer of robots. Then your car "calls ahead" and makes a reservation for every position in time and space it will occupy during the trip.
As long as you're covering the roads with Fog you may as well make it thick enough to hold the cars up so they can cross intersections at different levels. But now your car is no longer a specific set of robots, but a *pattern* in the road robots that moves along like a wave, just as a picture of a car moves across the pixels of a video screen. The appearance of the car at this point is completely arbitrary, and could even be dispensed with--all the road Fog is transparent, and you appear to fly along unsupported.
If you filled your house in with Fog this way, furniture no longer need be extruded from the floor; it can appear instantly as a pattern formed out of the "air" robots. Non-Fog objects can float around at will the way you did in your "car". But what's more, your surroundings can take on the appearance, and feel, of any other environment they can communicate with. Say you want to visit a friend; you both set your houses to an identical pattern. Then a Fog replica of him appears in your house, and one of you appears in his. The "air" fog around you can measure your actions so your simulacrum copies them exactly.
The pattern you both set your houses to could be anything, including a computer-generated illusion. In this way, Utility Fog can act as a transparent interface between "cyberspace" and physical reality.
Each Foglet has twelve arms, arranged as the faces of a dodecahedron. The central body of the foglet is roughly spherical, 10 microns in diameter. The arms are 5 microns in diameter and 50 microns long. A convex hull of the foglet approximates a 100-micron sphere. Each Foglet will weigh about 20 micrograms and contain about 5 quadrillion atoms. Its mechanical motions will have a precision of about a micron.
The arms telescope rather than having joints. The arms swivel on a universal joint at the base, and the gripper at the end can rotate about the arm's axis. The gripper is a hexagonal structure with three fingers, mounted on alternating faces of the hexagon. Two Foglets "grasp hands" in an interleaved six-finger grip. Since the fingers are designed to match the end of the other arm, this provides a relatively rigid connection; forces are only transmitted axially through the grip. When at rest, foglets form a lattice whose structure is that of a face-centered cubic crystal (i.e. an octet truss).
For a mass of Utility Fog to flow from one shape to another, or to exert dynamic forces (as in manipulating objects), a laminar flow field for the deformation is calculated. The foglets in each lamina remain attached to each other, but "walk" hand over hand across the adjacent layers. Although each layer can only move at a speed differential of 5 m/s with its neighbor, the cumulative shear rate in a reasonable thickness of Fog is considerable, up to 500 m/s per centimeter of thickness.
The atomically-precise crystals of the foglets' structural members will have a tensile strength of at least 100,000 psi. As an open lattice, the foglets occupy only about 3% of the volume they encompass. When locked in place, the Fog has a more or less anisotropic tensile strength of 1000 psi. In motion, this is reduced to about 500 if measured perpendicular to the shear plane. As a bulk material it has a density of 0.2 g/cc.
Without altering the lattice connectivity, Fog can contract by up to about 40% in any linear dimension, reducing its overall volume by a factor of five. (This is done by retracting all arms simultaneously.) Selective application of this technique allows Fog to simulate shapes and flow fields to a precision considerably greater than 100 microns.
An appropriate mass of Utility Fog can be programmed to simulate most of the physical properties of any macroscopic object (including air and water), to roughly the same precision those properties are measured by human senses. The major exceptions are taste, smell, and transparency. The latter an be overcome with holographic "eyephones" if a person is to be completely embedded in Fog.
Consider the application of Utility Fog to a task such as telepresence. The worksite is enclosed in a cloud of Fog, which simulates the hands of the operators to assemble the parts and manipulate tools. The operator is likewise completely embedded in Fog. Here, the Fog simulates the objects that are at the worksite, and allows the operator to manipulate them.
The Fog can also support the operator in such a way as to simulate weightlessness, if desired. Alternatively, the Fog at the worksite could simulate the effect of gravity on the objects there (in any desired direction).