Session M11: Drops: Freezing and Condensation

Freezing of Impacted Drops: Contact Line Dynamics

We experimentally explore the mechanisms of a water drop freezing while impacting a cold surface. The liquid drop spreads and partially freezes during the impact and then retracts upon ice. In the meantime the contact line is pinned thoughout a certain time of residence, unlike the case without solidification. Experimental measurements reveal non-trivial variations of this duration with the substrate thermal properties and drop impact velocity. A scaling law model enables us to interpret this phenomenon as the dynamic of the contact angle, which is affected by the solidification.
  

Freezing drop impact

We investigate experimentally the solidification of a water drop during its impact on a sub-zero cooled metallic plate. As the drop impacts the substrate, a first thin layer of ice builds-up in the briefest moment. Once this frozen disc is formed, the remaining liquid retracts on the top of it. The competition between this capillary retraction and the liquid solidification leads to a variety of frozen drop patterns. Typically, with a change of temperature the drop can freeze into a fried egg or a doughnut shape. Experimental measurements of the frozen drop profile, a 1D solidification model, and relevant scaling laws allow us to characterize the frozen disc thickness, and the different patterns, as a function of the control parameters.   

Solidification during impact: freezing the shape of a splash

The impact of a drop on an immiscible bath can generate different patterns depending on several parameters including the initial velocity of the drop and the density difference with the liquid bath. If the impacting drop can readily solidify upon cooling, the shape of the splash can be frozen in time. In this work, we study the morphologies obtained upon impacting a hot solution of paraffin wax in mineral oil on a cold immiscible bath. As it cools down, the impacting drop solidifies during the complex deformation processes associated with splashing, generating different morphologies that vary from cup-like to floral shapes with increasing number of petals. The anatomy of the final solidified wax is the result of a competition between different physical processes and instabilities that all occur over the time-scales of impact, including droplet breakup, wrinkling and crack formation /propagation in a thin stretched elastic sheet. Using high-speed imaging we show how different control parameters affect these fascinating morphologies that span the range from transient pattern formation in deforming fluids to the permanent indelible shape of thin elastic solid shells.   

Temperature measurements inside freezing drops in Poiseuille flow

Water is cooled down close to its melting point and is introduced through a syringe in the form of drops in an oil-continuous pipe flow. The oil has a temperature below that of the melting point of water and flows at a Reynolds number in the order of 10³. The heat transfer and solidification of the water drops is investigated downstream of the test section with planar laser-induced fluorescence (PLIF). A YLF laser sheet is emitted along the cross-stream direction of the flow and is used to harmonically excite a pair of fluorescent dyes with different emission spectra that are introduced in the water drop. Two cameras are connected to a beam-splitter and the fluorescence signal of each dye is captured in each camera individually. A ratiometric approach is used to accurately measure the temperature field inside the drop and account for reflection effects at the drop interface. The temperature gradients inside the drop and close to the thermal boundary layer are recorded for different Reynolds numbers and drop sizes, but also for different water and oil temperatures.