When the vessel containing the two liquids (cola on bottom, butane nearer the mouth) is inverted, the (relatively) warm water (or cola, or otherwise) comes in intimate contact with the liquid butane. The high surface area of the interaction causes the temperature of the butane to be immediately raised above 0°C, and is given plenty of energy to enter the gas phase. In the case of cola, I suppose it is possible that during all of this, droplets of butane are providing nucleation sites for CO2 as well, but as the whole effect can be shown to happen with water, it is rather beside the point.
When the vessel containing the two liquids (cola on bottom, butane nearer the mouth) is inverted, the (relatively) warm water (or cola, or otherwise) comes in intimate contact with the liquid butane. The high surface area of the interaction causes the temperature of the butane to be immediately raised above 0°C, and is given plenty of energy to enter the gas phase. In the case of cola, I suppose it is possible that during all of this, droplets of butane are providing nucleation sites for CO2 as well, but as the whole effect can be shown to happen with water, it is rather beside the point.
