Why do some powders form powder-filled blobs in a liquid?

Question

e.g., why does this happen? (I think in the picture, it's flour mixed with water. I've seen examples of this with many different powders mixed in liquids, usually water.)

What has happened at the boundary of the powder that made the powder-walls impenetrable to the fluid, so much so that the powder inside the blob is, by all appearances, as dry as if the liquid were not even present?

BONUS QUESTIONS:

1. What I think would be really cool, and impressive, is if someone can provide some mathematics to calculate, given a particular powder and a particular liquid at a particular temperature and at atmospheric pressure, the expected thickness of the blob-walls. (My intuition—unjustified for the time being—is that this thickness should not significantly depend on other factors than those I've just listed.)

2. Finally, one more piece of data associated with a slightly different line of inquiry: in my experience mixing powders and liquids, sometimes the blobs form and sometimes they do not. This seems to depend, among other things, on the manner in which the mixing happens. (For example, I have a collagen powder that I mix in my coffee every morning. I have noticed that the blobs do not usually form when I pour the powder on top of the coffee; in this case, the powder dissolves easily even without stirring. But when I pour the coffee on top of the powder and stir with a spoon, blobs galore!) This observation prompts the following question: what determines whether blobs actually form, or not?

Answer 1

Lumps in a sauce are the bane of many a cook, and you can find out how to avoid this issue at seasoned advice SE (the cooking SE site).

What happens, taking bleached wheat flour as an example, is best explained as a step-wise mechanism, since this is a kinetic, and not mainly a thermodynamic problem (if you mix the components long enough you should arrive at a homogeneous composition such as a a nice liquid batter, although this may phase separate if left unattended for longer times).

1. Flour is poured on the surface of the water. The flour is composed of small particles containing a mixture of amorphous and semi-crystalline regions of starch (amylose and amylopectin) and protein (including gluten).
2. The bottom surface of the flour in contact with water is wetted. Absorbed water causes swelling at the water boundary as starch polymer chains dissolve and expand. Water can infiltrate the boundary but is limited by slow diffusion through the gel.
3. Gravity draws the gel into the less dense solvent, dragging down the dry solid atop. Capillary forces cause water to encapsulate the dry-wet bilayer and prevent dispersion of the dry solid, engulfing the whole in solvent. Step 2 repeats for previously dry surfaces.
4. Air remains entrapped in the dry-wet blobs, providing buoyancy. Sufficiently wet blobs might become entirely submerged due to density differences.

The main point to explain the heterogeneity of the mixture is step 2. Swelling of starch and gel formation slows penetration of the solvent. If you have made a batter before, you might notice that leaving it standing typically leads to it becoming more viscous. This is due to water penetrating the starch granules reducing the volume of free water and causing the polysaccharide chains to swell.

I'd add that there are two issues here: one is movement of water into and through blobs, the second is dissolution of starch. Both movement of water through blobs and dissolution of starch are slow but persistent. Dissolution of starch is however much slower because in addition to being larger, polymer chains within a gel are entangled.