Dynamics in an interface can be disorienting, if not designed appropriately. Viscosity is generally defined as resistance to flow. In the context of graphical user interfaces, however, a viscous interface allows the users to make easy associations between consecutive dynamically changing displays with little cognitive overhead, where users can easily see the continuity without any disorientation.
Typically, many factors affect the viscosity of an interface where most of them can be examined in the context of the layout functions in the presentation hierarchy: speed, contents, size, direction, and extent.
Speed at which changes in the spatial properties of a node instance take place affects viscosity considerably. It can easily be argued that if node instances are relocated too far off from their initial position, it is less likely that users can make associations between the consecutive displays. The richness of contents and the size of node instances can be good discriminators. Direction of movement is also critical. If node instances tend to move all in different directions as opposed to a coherent move (e.g. in the same direction in parallel, in radially outwards/inwards directions, etc.), it is harder to understand which node instance is moving where. The extent of the dynamics in the screen space determines the area at which the user has to pay attention to, thus affects viscosity. Dynamics that affect a large portion of the screen space are argued to be more disorienting.
The WebBook and the Web Forager interface demonstrate a number of dynamics. Page shuffling is argued to be a viscous dynamics in regards to all of the above factors. Speed is at an acceptable rate, which allows users to see enough of the contents. Direction of page movements are all following the same pattern, within the screen space limits of the book instance which is the extent of the page shuffling dynamics. Other dynamics such as movement of pages to and from the air, the shelves, and the desk occur all under user control, and affect only a single node instance. Thus, they are also considered fairly viscous.
The vtwm is not very rich in regards to dynamics, besides the single window move and resize operations. These operations also are under user control, and thus argued to be viscous.
Elastic layout dynamics in the Elastic Windows interface is a viscous dynamics which allows users to resize and move groups of windows. The size and direction of windows change under user control as in an elastic material. In the current implementation, when a layout being changed only windows borders are drawn. Window contents are only drawn upon completion of the operation. This decreases viscosity considerably. However, as a well-known metaphor, it is easily understandable and its effects on the size and location of windows are typically as expected. The extent of the dynamics is the space of the parent window, thus only windows within that group are affected, contributing to the viscosity of the interface.