The downward motion is often liquid or small bubbles, not the bubble sinking
When people ask whether bubbles splash downward, the image is usually a bubble diving into liquid. The experiments describe something more specific: a bubble's growth or collapse pushes surrounding liquid and nearby bubbles around. In one study, a cavitation bubble growing above a small hole drove a downward flow that compressed an air bubble trapped at the base of the hole [3]. That downward flow is liquid motion created by the bubble, not the cavitation bubble itself settling downward.
A separate pool experiment reports small bubbles moving downward in the near-wall region, alongside splashing droplets formed when liquid columns or the edges of bursting bubbles tear apart [2]. So downward-moving bubbles do appear in observations, but they are small bubbles near a wall, and the abstract does not establish that they are the same bubbles that produced the splash. The useful distinction is between a bubble that travels down and a bubble that causes something else to move down.
Walls and confinement decide which way the motion goes
A bubble collapsing in open liquid tends to behave symmetrically, but a nearby wall breaks that symmetry. In the semi-cylindrical groove study, the bubble's position relative to the groove and flat wall produced different outcomes: it could migrate from the groove toward the flat wall, impact the flat wall vertically downward, or migrate from the flat wall back to the groove [1]. The downward impact is therefore one of several possible paths, not a universal rule.
The confined-air-bubble experiment shows the same sensitivity from another angle. Whether a vortex ring formed at all depended on the bubble's stand-off distance from the hole and how much of the hole the air bubble filled; ring formation occurred only in a middle timing window, while late impacts and direct air-bubble impacts produced no ring [3]. Direction and structure of the resulting flow are set by geometry and timing, which is why a simple yes-or-no answer misses the physics.
The downward push is strongest during collapse and close to the wall
The clearest downward effect in the supplied evidence comes from a collapsing cavitation bubble interacting with a confined air bubble. As the cavitation bubble grows above the hole, it drives a downward flow that compresses the trapped air bubble; that air bubble then expands and expels the overlying liquid column upward [3]. The downward phase is part of a cycle, and the later upward slug can be just as important as the initial push.
The groove study adds that the downward impact on the flat wall appears among asymmetric bubble positions, where the bubble is not centered on the groove [1]. That places the effect in a particular geometric situation rather than in every bubble collapse. The pool observations likewise locate downward-moving small bubbles in the near-wall region [2], consistent with walls shaping the flow, though the abstract does not explain the mechanism behind that motion.
What the evidence does not yet settle
The supplied abstracts do not give a single measured downward splash from a free bubble in open water. The groove paper reports a bubble impacting a flat wall vertically downward as one migration case [1], and the confined-bubble paper reports a downward flow that compresses an air bubble [3], but neither abstract quantifies how much liquid is carried downward or how far. The pool paper reports downward movement of small bubbles near a wall without stating what drives it [2].
So the honest answer is that downward motion is real and reproducible in these wall-influenced setups, but its strength and form depend on stand-off distance, confinement, and collapse timing [1][3]. A reader should not generalize from these experiments to every bubble in every liquid. The direction can reverse within the same event, as when the downward compression is followed by an upward liquid slug [3].
Sources used in this answer
Research on the Collapse Characteristics of Cavitation Bubbles near a Semi-Cylindrical Groove
Near a semi-cylindrical groove, a cavitation bubble can migrate toward a flat wall and impact it vertically downward, one of several position-dependent collapse outcomes.
Experimental study on pool two-phase flow and droplet entrainment characteristics
In pool two-phase flow, small bubbles were observed moving downward in the near-wall region, and splashing droplets formed when liquid columns or bursting-bubble edges tore apart.
Vortex ring formation from the interaction of a cavitation bubble with a confined air bubble: experiments and a timing criterion
A growing cavitation bubble above a blind hole drives a downward flow that compresses a confined air bubble, which then expels liquid upward and can form a vortex ring only within a limited timing window.