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Opposition to movement is a multi-fold process. The surfaces of encounter between the swimmer and the fluid provide resistance to efficient motion. Energy is sucked off as fluids change from laminar to turbulent flow. Flow may also become diverted inward creating rotations called vortices that project downstream from the swimmer. Drag depends on inertia (the tendency to stay put). Inertia is proportional to mass. Fluid density or mass per unit volume is the important factor …

…not end up next to each other at the trailing edge. Why should they? Molecules have no "knowledge" of their neighbors - they're inanimate. In fact, molecules are in constant random motion. And two molecules near each other at any instance will, in all probability, never be near each other again - even in still air. The part of the theory about Bernoulli's equation and a difference in pressure existing across the airfoil is correct.