Thicker liquids like honey, coaltar, glycerine, etc. (i) A stirred liquid, when left, comes to rest on account of viscosity. The property of viscosity is seen in the following examples: The property of the liquid by virtue of which it opposes the relative motion between its adjacent layers is known as viscosity. In the absence of the external force, the viscous forces would soon bring the liquid to rest. If the flow of the liquid is to be maintained, an external force must be applied to overcome the dragging viscous forces. These forces are called ‘viscous forces’. This means that in between any two layers of the liquid, internal tangential forces act which try to destroy the relative motion between the layers. Similarly, each layer tends to increase the velocity of the layer below it. Thus each layer tends to decrease the velocity of the layer above it. The layer a tends to retard the layer b, while b tends to retard c. Their velocities are in the increasing order. Let us consider three parallel layers a, b and c. Thus there is a relative motion between adjacent layers of the liquid. In the figure, the lengths of the arrows represent the increasing velocity of the layers. The layer of the liquid which is in contact with the surface is at rest, while the velocity of other layers increases with distance from the fixed surface. Suppose a liquid is flowing in streamlined motion on a fixed horizontal surface AB. This force is called ‘internal frictional-force’. Similarly, when a layer of a liquid slides over another layer of the same liquid, a frictional-force acts between them which opposes the relative motion between the layers. This force opposes the relative motion of the bodies. The viscous liquids (honey) build up overpressure more easily and so are more explosive and bubbles need more time to rise to the surface because of the resistance of the melt.When a solid body slides over another solid body, a frictional-force begins to acts between them. First of all, the video shows that viscosity increases with decreasing temperature. This video by Barry Bickmore shows the effect of viscosity on bubbles with experiences with water, honey and hot honey. We can also observe too small bubbles which are reabsorbed in the melt. This video shows the nucleation of bubbles and their ascension with growth. It’s very important to understand the feedback effect of bubbles: as bubbles nucleate and grow, viscosity increases which leads to a much greater potential energy, though to a very violent eruption. Because of the resistance of the melt, bubbles can’t expand and an internal overpressure appears. The second stage of bubbles growth is limited by viscosity. The violence of explosions is greatest in the most viscous magmas.įor instance, in Hawaii, the basaltic magma of the Kilauea products only lava flows whereas the more viscous magma of the Mount Pinatubo produced one of the largest eruption in 1991. Because of the initial composition and volatile content, rhyolite becomes more and more viscous whereas basalt stays low viscous. When bubbles become major proportions of the bulk volume, the bulk material experiences increased viscosity, which can lead to potential energy storage and then to explosive eruptions. 6.6 Bubbles induce an increase of the melt viscosity
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