You're confusing buoyancy with surface tension. The former applies to (macroscopic) objects which are (at least partly) immersed, while the latter applies to inter-fluid surfaces.

Thus, a sufficiently small grain of rock (sand) may stay on the water surface owing to its tension, but will sink if submerged (say by a breaking wave). If you make it very small, however, it will undergo Brownian motion (when submerged) and the fluid model (which describes buoyancy) is no longer applicable.

In answer to the comment: if submerged, it's (by definition) underneath the surface: surface tension is out of the equation and buoyancy rules. In addition, there is of course a frictional drag force between fluid and object (which depends on the shape and speed of the object, as well as the viscosity of the fluid). After some initial transient period, the drag force, gravity, and buoyancy become balanced and the object sinks at a constant rate, which doesn't depend on the object's size but only its shape.

You're confusing buoyancy with surface tension. The former applies to (macroscopic) objects which are (at least partly) immersed, while the latter applies to inter-fluid surfaces.

Thus, a sufficiently small grain of rock (sand) may stay on the water surface owing to its tension, but will sink if submerged (say by a breaking wave). If you make it very small, however, it will undergo Brownian motion (when submerged) and the fluid model (which describes buoyancy) is no longer applicable.

In answer to the comment: if submerged, it's (by definition) underneath the surface: surface tension is out of the equation and buoyancy rules. In addition, there is of course a frictional drag force between fluid and object (which depends on the size, shape, and speed of the object, as well as the viscosity and density of the fluid). After some initial transient period, the drag force, gravity, and buoyancy become balanced and the object sinks at a constant rate, which does depend on the object's mass, size, and shape.

You're confusing buoyancy with surface tension. The former applies to (macroscopic) objects which are (at least partly) immersed, while the latter applies to inter-fluid surfaces.

Thus, a sufficiently small grain of rock (sand) may stay on the water surface owing to its tension, but will sink if submerged (say by a breaking wave). If you make it very small, however, it will undergo Brownian motion (when submerged) and the fluid model (which describes buoyancy) is no longer applicable.

You're confusing buoyancy with surface tension. The former applies to (macroscopic) objects which are (at least partly) immersed, while the latter applies to inter-fluid surfaces.

Thus, a sufficiently small grain of rock (sand) may stay on the water surface owing to its tension, but will sink if submerged (say by a breaking wave). If you make it very small, however, it will undergo Brownian motion (when submerged) and the fluid model (which describes buoyancy) is no longer applicable.

In answer to the comment: if submerged, it's (by definition) underneath the surface: surface tension is out of the equation and buoyancy rules. In addition, there is of course a frictional drag force between fluid and object (which depends on the shape and speed of the object, as well as the viscosity of the fluid). After some initial transient period, the drag force, gravity, and buoyancy become balanced and the object sinks at a constant rate, which doesn't depend on the object's size but only its shape.