Cinnamon soap rush

Panel 1, "BEFORE: stretched skin". On the left is the plate: a thin layer of water with cinnamon floating on top. The circle is a magnifier, and it zooms way, way in, all the way to the water molecules ("zoom: water molecules"). A cinnamon speck would be enormous at that scale, so it is left out of the zoom.

Each blue ball is a water molecule. Water molecules pull on the neighbours right next to them. Notice the top row is just as jumbled as the rest. There is no special tidy layer at the surface.

Look at the one marked "inside: balanced". It has neighbours all around it, so its pull arrows go every way and cancel out.

Now look at the one right at the top, "surface: none above, net pull in". It has neighbours beside it and below it, but none above (that's air). So its pulls don't cancel. The sideways ones balance each other, and what is left over is a pull inward, into the water: the bigger arrow down.

Every molecule along the top is like that: tugged inward, with fewer friends than the ones below. (The molecules underneath push back, so nothing sinks.) So water tries to keep as few molecules at the surface as it can. It pulls its surface as small and tight as possible, like a stretched skin ("acts like a stretched skin"). That tightness is surface tension. Floating cinnamon just sits on that skin.

Panel 2, "SOAP: the skin pulls away". On the left is the plate seen from above, a second after the soap touch. The green dot in the middle is the soap. The dashed arrows point outward from it: the surface is moving out, and the cinnamon has already been carried out toward the rim.

The window on the right is a side view cut straight through the soap spot: soap molecules in the middle, clean water on both sides. The green balls with tails sitting on the surface are "soap molecules". Soap is a surfactant: it sits at the surface and weakens the pull there.

Now look at the two black squares, one at each edge of the soap patch, right at the soap front. Each one is a little bit of surface caught in a tug of war. The short solid arrow pointing back over the soap is the pull from the soapy side: "weak". The long solid arrow pointing out is the pull from the clean water: "strong". Strong wins, so the surface slides away from the soap spot, on both sides.

The dashed arrows under the surface are the moving surface dragging the water just under it along: "surface slides out, dragging water". The cinnamon rides on top of that moving surface, all the way to the edges. Nothing pushes the cinnamon: not air, not the soap. The water's surface moves and carries it. (Where does that water come from? It wells up from underneath the soap spot, and a slow return flow creeps back along the bottom.)