Why a small Bond number doesn't rule out gravity
The Bond number compares gravitational forces to surface tension forces. A small Bond number means surface tension dominates gravity in determining the droplet's overall shape, so the droplet stays nearly spherical. But this does not mean gravity is irrelevant for the flow inside. Gravity can still drive motion if density differences within the liquid create buoyancy forces that overcome viscous resistance [1].
In an evaporating droplet made of two liquids with different volatilities, the more volatile component (like water in a glycerol-water mixture) evaporates faster. This leaves behind a higher concentration of the less volatile component near the evaporating surface, creating a density gradient. Gravity then pulls on the denser regions, generating convective flow. This effect can occur even when the droplet is sub-millimeter and the Bond number is much less than one [1].
The Archimedes number tells you when gravity matters
The Archimedes number (Ar) measures the ratio of buoyancy forces to viscous forces. When Ar is larger than about one, buoyancy-driven convection becomes important; when Ar is smaller than one, viscous forces suppress it. In experiments with glycerol-water droplets, the flow direction reversed between sessile (sitting on a surface) and pendant (hanging) droplets—inward versus outward radial flow—which is a clear signature of gravity acting in opposite directions relative to the droplet [1].
By changing the droplet height and using different liquid mixtures (glycerol-water versus 1,2-propanediol-water), researchers could tune Ar to be either above or below one. The gravitational effects appeared only when Ar exceeded one, confirming that Ar, not the Bond number, is the proper criterion for deciding whether to include gravity in the flow model [1].
Gravity can also matter for shape and solute transport
Beyond composition-driven convection, gravity can influence other aspects of evaporating droplets. For example, in droplets containing suspended particles that form an elastic film during evaporation, the direction of gravity affects the final morphology: sessile droplets crumple near the apex while pendant droplets wrinkle near the contact line. A model that includes gravity and elasticity shows that gravity's direction remains crucial even for very small droplets where it is normally neglected [5].
Gravity also affects where solutes end up in an evaporating droplet. In a sessile droplet, gravity can alter the flow that carries particles to the edge, influencing coffee-ring formation. Analyses that include gravity show that even when the Bond number is small, gravitational effects on the flow can be significant under certain conditions, particularly when density gradients arise from evaporation [2].
Sources used in this answer
Gravitational effect in evaporating binary microdroplets
In evaporating glycerol-water microdroplets with Bond number much less than one, gravity drives opposite radial flow directions in sessile versus pendant droplets, and the Archimedes number determines whether gravitational effects occur.
Gravitational effects on coffee-ring formation during the evaporation of sessile droplets
Gravity can influence solute transport and coffee-ring formation in evaporating sessile droplets, including in regimes where the Bond number is small.
Hydrothermal waves on ethanol droplets evaporating under terrestrial and reduced gravity levels
Hydrothermal waves on evaporating ethanol droplets are studied under terrestrial and reduced gravity, but the paper does not address small Bond number scenarios.
Dynamics of weakly evaporating and non-evaporating drops falling in air
The dynamics of evaporating and non-evaporating drops falling in air under gravity are experimentally and theoretically studied, but the paper does not specifically address small Bond number flows inside droplets.
Flattened and Wrinkled Encapsulated Droplets: Shape Morphing Induced by Gravity and Evaporation
In evaporating droplets with an encapsulating elastic film, gravity's direction controls whether sessile droplets crumple near the apex or pendant droplets wrinkle near the contact line, even for very small droplets.