Controlled Multibubble Surface Cavitation

Abstract

Heterogeneous bubble nucleation at surfaces has been notorious because of its irreproducibility. Here controlled multibubble surface cavitation is achieved by using a hydrophobic surface patterned with microcavities. The expansion of the nuclei in the microcavities is triggered by a fast lowering of the liquid pressure. The procedure allows us to control and fix the bubble distance within the bubble cluster. We observe a perfect quantitative reproducibility of the cavitation events where the inner bubbles in the two-dimensional cluster are shielded by the outer ones, reflected by their later expansion and their delayed collapse. Apart from the final bubble collapse phase (when jetting flows directed towards the cluster's center develop), the bubble dynamics can be quantitatively described by an extended Rayleigh-Plesset equation, taking pressure modification through the surrounding bubbles into account.

Superstability of Surface Nanobubbles

“…Combining the cavitation and AFM experiments allows to study the relation between cavitation activity and nanobubbles. An analogous strategy has been used previously [20] to perfectly correlate the appearance of surface bubbles in cavitation experiments to the existence of gas-filled microcavities (i.e., microbubbles) of diameter of 2 − 4 µm which had been etched into the surface. Is there a similar connection between cavitation on smooth unstructured surfaces and surface nanobubbles?…”

“…The cavitation event is photographed by a CCD camera through a long-distance microscope. The major difference between the present setup and that of [18,19,20] is the maintenance of cleanroom conditions by use of the protective flask. Compared to Fig.…”

“…The setup for the cavitation experiments is similar to that used in [18,19,20]. A shock wave generator generates a pressure signal in the water, consisting of a high pressure front followed by a large tensile stress, see Fig.…”

“…Cavitation bubbles originate from contaminations or from microscopic structures such as microcracks or microcrevices [19,20], rather than from surface nanobubbles which densely populate the immersed substrates. This implies that surface nanobubbles are unexpectedly stable under large tensile stresses.…”

“…More precisely, cavitation experiments (similar to [18,19,20]) with different hydrophobic substrates submerged in water are performed: a shock wave generates a large tensile stress (≈ −6 MPa) in the water which leads to cavitation of bubbles at the substrates. The size of the cavitation bubbles is of the order of several hundred µm.…”