Oscar R. Enríquez

Universality of Tip Singularity Formation in Freezing Water Drops

A drop of water deposited on a cold plate freezes into an ice drop with a pointy tip. While this phenomenon clearly finds its origin in the expansion of water upon freezing, a quantitative description of the tip singularity has remained elusive. Here we demonstrate how the geometry of the freezing front, determined by heat transfer considerations, is crucial for the tip formation. We perform systematic measurements of the angles of the conical tip, and reveal the dynamics of the solidification front in a Hele-Shaw geometry. It is found that the cone angle is independent of substrate temperature and wetting angle, suggesting a universal, self-similar mechanism that does not depend on the rate of solidification. We propose a model for the freezing front and derive resulting tip angles analytically, in good agreement with the experiments.

Growing bubbles in a slightly supersaturated liquid solution

We have designed and constructed an experimental system to study gas bubble growth in slightly supersaturated liquids. This is achieved by working with carbon dioxide dissolved in water, pressurized at a maximum of 1 MPa and applying a small pressure drop from saturation conditions. Bubbles grow from hydrophobic cavities etched on silicon wafers, which allows us to control their number and position. Hence, the experiment can be used to investigate the interaction among bubbles growing in close proximity when the main mass transfer mechanism is diffusion and there is a limited availability of the dissolved species.

Collapse and pinch-off of a non-axisymmetric impact-created air cavity in water

The axisymmetric collapse of a cylindrical air cavity in water follows a universal power law with logarithmic corrections. Nonetheless, it has been suggested that the introduction of a small azimuthal disturbance induces a long-term memory effect, reflecting in oscillations which are no longer universal but remember the initial condition. In this work, we create non-axisymmetric air cavities by driving a metal disc through an initially quiescent water surface and observe their subsequent gravity-induced collapse. The cavities are characterized by azimuthal harmonic disturbances with a single mode number and amplitude . For small initial distortion amplitude (1 or 2 % of the mean disc radius), the cavity walls oscillate linearly during collapse, with nearly constant amplitude and increasing frequency. As the amplitude is increased, higher harmonics are triggered in the oscillations and we observe more complex pinch-off modes. For small-amplitude disturbances we compare our experimental results with the model for the amplitude of the oscillations by Schmidt et al. (Nature Phys., vol. 5, 2009, pp. 343?346) and the model for the collapse of an axisymmetric impact-created cavity previously proposed by Bergmann et al. (J. Fluid Mech., vol. 633, 2009b, pp. 381?409). By combining these two models we can reconstruct the three-dimensional shape of the cavity at any time before pinch-off.

Collapse of nonaxisymmetric cavities

A round disk with a harmonic disturbance impacts on a water surface and creates a non-axisymmetric cavity which collapses under the influence of hydrostatic pressure. We use disks deformed with mode m=2 to m=6. For all mode numbers we find clear evidence for a phase inversion of the cavity wall during the collapse. We present a fluid dynamics video showing high speed imaging of different modes, pointing out the characteristic features during collapse.

Non-axisymmetric impact creates pineapple-shaped cavity

We impact a disk on a free water surface at a controlled speed of 1 m=s. The disk is round, with a superimposed mode-20 azimuthal disturbance. The mean disk radius is 20 mm and the amplitude of the disturbance is 0.4 mm. Initially, very close to the disk, the free surface is forced to match the shape of the disk. During the void expansion and subsequent collapse, however, the interface displays rich dynamics, resulting eventually in a pineapple-shaped cavity. If we made a cut-through of the cavity at one specific depth, we would observe an oscillating behavior of the water-air interface just like a standing wave coupled to the fast decreasing mean radius of the cavity. The amplitude of this oscillation remains constant, while the frequency diverges towards the pinch-off—following the prediction made by linear stability analysis of a disconnecting air bubble. Since the absolute amplitude remains constant while the mean radius of the cavity goes to zero, the relative amplitude grows strongly towards the pinch-off; the disturbance thus becomes much more pronounced closer to the pinch-off (e.g., compare Fig. 1(b) with 1(c)). Since the radial flow in this system is much larger than the axial flow, we can approximate each horizontal layer of fluid as being decoupled from the vertical direction. It is, therefore, possible to solve the system at each layer by combining the radial dynamics of an axisymmetric cavity with the model for the oscillations. This was done by Enrı́quez et al., resulting in an almost perfect reproduction of the full pineapple-shaped cavity.

Deactivation of Microbubble Nucleation Sites by Alcohol! Water Exchange

The ethanol-water exchange process is one of the standard methods of generating nanobubbles at a solid-water interface. In this work, we examine whether the nanobubbles formed by the solvent exchange can initiate microbubble formation as the temperature increases, thus acting as nuclei. This, however, is not the case: the nanobubbles are stable and do not facilitate microbubble formation. Instead, the process of solvent exchange, which aids the formation of nanobubbles and even microbubbles on some hydrophobic substrates under ambient conditions, suppresses microbubble nucleation on graphite and hydrophilic micropit-decorated substrates at high temperature (i.e., deactivates the nucleation sites for microbubble formation). We ascribe this behavior to the prewetting of the surface by the alcohol and the stability of the nanobubbles to the temperature increase. The findings in this study have implications for the prevention of bubble formation for a range of applications.