While drop oscillation dynamics has been widely studied for many decades, the influence of a moving contact line on the oscillation modes of drops remains underexplored. Herein, we report the oscillation dynamics of drops on thin liquid films with different viscosities where lower viscosities provide a slipping surface and higher viscosities immobilize the contact line. A gently deposited drop onto an oil film undergoes shape oscillations due to capillarity, where the frequency, amplitude, and apparent contact angle are tracked via a high-speed camera. This study demonstrates that restraining the mobility of the drop contact line by increasing the viscosity of a thin oil film underneath the drop increases the extent of the drop oscillation time as well as affecting the natural frequency of the drop oscillation. The drop oscillation time was defined by the time at which the changes in the drop height dropped to values less than 1% of the equilibrium height. The experimental results for the first longitudinal mode oscillation frequencies as a function of the equilibrium contact angles for the pinning and slipping contact lines were in good agreement with previously reported numerical simulations and model predictions.
Recent studies have revealed the air-cushioning effect of droplet impact upon various surfaces and although pure water droplets have been studied extensively, the air entrainment dynamics for aqueous polymeric droplets is the focus of this study. Herein, we show that for low to moderate Weber numbers, We ~ O(1 - 10), the air film thickness gradient is strongly influenced by the viscoelastic properties of the aqueous polymeric droplets in the dilute to the semidilute unentangled regimes. Droplets of aqueous polyethylene oxide (PEO) impacting a smooth thin oil film surface formed a submicron air layer moments prior to impact, which was tracked by a high-speed total internal reflection microscopy (TIRM) technique. The radial changes in the air film thickness were related to the polymer concentration, thus providing an alternative tool for comparing the rheometer-derived overlap concentrations with a contactless optical technique.
presented for the various drop impact regimes for the thin (i.e., O(1 μm))and thick (i.e., O(100 μm)) liquid film thickness limits. The incidence of the contact bouncing phenomenon was also characterized and found to be intimately tied to the late-stage gas film failure where a small volume of liquid is deposited onto the liquid film prior to bouncing. This study sheds new light on the bouncing-merging transitions for drops on ultrathin liquid films and gas entrainment dynamics relevant to applications such as precise drop deposition techniques.
Volatile drop impacts are commonplace among various industrial and natural processes and most often studied under Leidenfrost conditions, where a vaporized film sustains the drop weight or reverses drop momentum. The vapor thrust generated is therefore a function of the enthalpy of vaporization, the superheat, specific heat capacity of the vapor, vapor thermal diffusivity, and the vapor film thickness. In this study, volatile drop impact and wetting dynamics of acetone and isopropanol mixtures at normal temperature and pressures were shown to generate enough thrust from evaporation alone during the impact process and allow for unique contact dynamics. Volatility was controlled by varying the acetone concentration in isopropanol mixtures (O(1 - 10 kPa) to keep surface tension relatively constant while the vapor pressure and viscosity increased. Nucleation onset was tracked using a high-speed optical total internal reflection microscopy (TIRM) technique where an increase in the vapor pressure enhanced wetting onset (i.e., pure acetone). However, the concentrations between 49 - 66% isopropanol which have vapor pressures of kPa, respectively, caused droplets to rebound at We up to We 21, beyond the classic disjoining pressure dominant regime of We > 10.
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