As the liquid and foam that collects on the back of the spoon sits waiting for the next dip in the liquor, liquid draining off across the silver, the bubbles begin losing internal cohesion and increase in size, number and spacing. It’s also likely that the foam is dehydrating some. You can see – beginning near the edges and moving inwards – how the liquid begins to show signs of drying and thickening as it clings to the surface with increased urgency.
If nothing else, this points out how quickly it changes.
*This isn’t crema, rather it’s the foam that forms on the surface of the coffee liquor after steeping. Times are approximated from adding water. 
