E.g. tomato sauce
A fluid gel is something that is a gel when sitting quietly, but that transforms into a liquid when sheared by some means (blended, agitated, masticated), but that can then reverse back into a gel when rested again.
A typical way to turn a liquid into a fluid gel is to add Xanthan gum (E-415) to it. This gum is a carbohydrate derived from fermenting cornstarch with the bacterium Xanthomonas campestris (as found in cabbage).
Sounds implausible? Here are two examples:
Example 1: Bottled salad dressing. Particularly those ‘French’ or ‘Italian’ dressings with chopped herbs suspended in the jar. Without provoking calculations of my age, when I was a lad, the herbs always settled to the bottom of the jar. Their modern manufacture now as a fluid gel means that while the jar is sitting quietly on the shelf the dressing takes the form of a gel, and the herbs are suspended in it in an attractive way that might tempt purchase.
When it comes time to use it, the jar is shaken, transforming the gel into a liquid that can be poured onto the salad.
The salad leaves are dressed with the liquid, and tossing the salad maintains the dressing in its liquid form, helping to evenly coat the leaves.
When it is plated, the salad comes to rest and the dressing returns to its gel form. Which means it sticks it to the salad leaves, rather than dripping off into a puddle on the bottom of the plate.
During eating, the gel is transformed back into a liquid as a result of agitation from chewing — the result is a nice combination of salad and a liquid dressing.
Example 2: Tomato sauce. You know this — it won’t come out of the bottle — shake — comes out (usually too fast). Same as the salad dressing — it rests where it lands and clings comfortably on the pie/sausage roll, to be eaten without dripping off.
So there you have it — you’ve probably been eating Xanthan gum and fluid gels for most of your life.