At impact of a liquid drop on a solid surface an air bubble can be entrapped. Here we show that two competing effects minimize the (relative) size of this entrained air bubble: For large drop impact velocity and large droplets the inertia of the liquid flattens the entrained bubble, whereas for small impact velocity and small droplets capillary forces minimize the entrained bubble. However, we demonstrate experimentally, theoretically, and numerically that in between there is an optimum, leading to maximal air bubble entrapment. Our results have a strong bearing on various applications in printing technology, microelectronics, immersion lithography, diagnostics, or agriculture.The impact of liquid droplets on surfaces is omnipresent in nature and technology, ranging from falling raindrops to applications in agriculture and inkjet printing. The crucial question often is: How well does the liquid wet a surface? The traditional view is that it is the surface tension which gives a quantitative answer. However, it has been shown recently that an air bubble can be entrapped under a liquid drop as it impacts on the surface [1-6]. Also Xu et al. [7,8] revealed the important role of the surrounding air on the impact dynamics, including a possible splash formation. The mechanism works as follows [3][4][5][6]: The air between the falling drop and the surface is strongly squeezed, leading to a pressure buildup in the air under the drop. The enhanced pressure results in a dimple formation in the droplet and eventually to the entrapment of an air bubble (figure 1a). The very simple question we ask and answer in this paper is: For which impact velocity is the entrapped bubble maximal?Our experimental setup is shown in figure 1b and is similar to that of ref.[9] where it is described in detail. An ethanol drop impacts on a smooth glass surface after detaching from a needle, or for velocities smaller than 0.32 m/s, after moving the needle downwards using a linear translation stage. A high-speed side view recording is used to measure the drop diameter and velocity. A synchronized bottom view recording by a high-speed color camera is used to measure the deformed shape of the liquid drop. Colored interference patterns are created by high-intensity coaxial white light, which reflects from both the glass surface and the bottom of the droplet. Using a color-matching approach in combination with known reference surfaces, the complete air thickness profile can be extracted (shown in figure 1c). For experiments done at larger impact velocities (U > 0.76 m/s), we use a pulse of diffused laser light triggered by an optical switch. The thickness of the air film at the rim is assumed to be zero, and the complete air thickness profile can then be obtained from the monochromatic fringe pattern. From these measurements we can determine the dimple height, H d , and the volume of the entrained bubble, V b , at the very moment of impact. This moment is defined by the first wetting of the surface, i.e., the moment when the concentric symmetry of th...

The ubiquity of turbulent flows in nature and technology makes it of utmost importance to fundamentally understand turbulence. Kolmogorov's 1941 paradigm suggests that for strongly turbulent flows with many degrees of freedom and its large fluctuations, there would only be one turbulent state as the large fluctuations would explore the entire higher-dimensional phase space. Here we report the first conclusive evidence of multiple turbulent states for large Reynolds number Re = O(10 6 ) (Taylor number Ta = O(10 12 )) Taylor-Couette flow in the regime of ultimate turbulence, by probing the phase space spanned by the rotation rates of the inner and outer cylinder. The manifestation of multiple turbulent states is exemplified by providing combined global torque and local velocity measurements. This result verifies the notion that bifurcations can occur in high-dimensional flows (i.e. very large Re) and questions Kolmogorov's paradigm.

In the Leidenfrost effect, liquid drops deposited on a hot surface levitate on a thin vapor cushion fed by evaporation of the liquid. This vapor layer forms a concave depression in the drop interface. Using laser-light interference coupled to high-speed imaging, we measured the radius, curvature, and height of the vapor pocket, as well as nonaxisymmetric fluctuations of the interface for water drops at different temperatures. The geometry of the vapor pocket depends primarily on the drop size and not on the substrate temperature.

A drop impacting on a solid surface deforms before the liquid makes contact with the surface. We directly measure the time evolution of the air layer profile under the droplet using high-speed color interferometry, obtaining the air layer thickness before and during the wetting process. Based on the time evolution of the extracted profiles obtained at multiple times, we measure the velocity of air exiting from the gap between the liquid and the solid, and account for the wetting mechanism and bubble entrapment. The present work offers a tool to accurately measure the air layer profile and quantitatively study the impact dynamics at a short time scale before impact.

In the maritime industry, the injection of air bubbles into the turbulent boundary layer under the ship hull is seen as one of the most promising techniques to reduce the overall fuel consumption. However, the exact mechanism behind bubble drag reduction is unknown. Here we show that bubble drag reduction in turbulent flow dramatically depends on the bubble size. By adding minute concentrations (6 ppm) of the surfactant Triton X-100 into otherwise completely unchanged strongly turbulent Taylor-Couette flow containing bubbles, we dramatically reduce the drag reduction from more than 40% to about 4%, corresponding to the trivial effect of the bubbles on the density and viscosity of the liquid. The reason for this striking behavior is that the addition of surfactants prevents bubble coalescence, leading to much smaller bubbles. Our result demonstrates that bubble deformability is crucial for bubble drag reduction in turbulent flow and opens the door for an optimization of the process.Theoretical, numerical and experimental studies on drag reduction (DR) of a solid body moving in a turbulent flow have been performed for more than three decades [1][2][3][4][5][6]. A few volume percent (≤ 4%) of bubbles can reduce the overall drag up to 40% and beyond [7][8][9][10][11][12][13][14]. However, the exact physics behind this drag reduction mechanism is unknown, thus hindering further progress and optimization, and even the dependence of the effect on the bubble size is controversial [15][16][17], though it is believed to be independent of the bubble size [1].In this Letter, we experimentally investigated the mechanism behind bubble drag reduction in a TaylorCouette (TC) system, i.e. the flow between two independently rotating coaxial cylinders. The TC system can be seen as "drosophila" of physics of fluids, with many concepts in fluid dynamics being tested therewith, ranging from instabilities, to pattern formation, to turbulence, see the reviews [18,19]. Here we inject bubbles into the system, which due to the density difference to water experience a centripetal force towards the inner cylinder, mimicking the upwards gravitational force acting on bubbles under a ship hull.The experiments are performed in the Twente Turbulent Taylor-Couette facility (T 3 C) [20], with the inner one strongly rotating, corresponding to very large Reynolds number of Re ∼ 10 5 − 10 6 . The setup has an inner cylinder with a radius of r i = 200 mm and an outer cylinder with a radius of r o = 279 mm, resulting in a radius ratio of η = r i /r o = 0.716. The inner cylinder rotates with a frequency up to f i = 20 Hz, resulting in Reynolds numbers up to Re = 2πf i r i (r o − r i )/ν α = 2 × 10 6 , in which ν α is kinematic viscosity of water-bubble mixture. The outer cylinder is at rest. The cylinders have a height of L = 927 mm, resulting in an aspect ratio of Γ = L/(r o − r i ) = 11.7. The flow is cooled through both endplates to prevent viscous heating through the viscous dissipation. The torque τ is measured with a co-axial torque transducer (Ho...

The dissolution dynamics of microscopic oil droplets (less than 1 μm in height, i.e. nanodroplets) on a hydrophobilized silicon surface in water was experimentally studied. The lateral diameter was monitored using confocal microscopy, whereas the contact angle was measured by (disruptive) droplet polymerisation of the droplet. In general, we observed the droplets to dissolve in a mixed mode, i.e., neither in the constant contact angle mode nor in the constant contact radius mode. This means that both the lateral diameter and the contact angle of the nanodroplets decrease during the dissolution process. On average, the dissolution rate is faster for droplets with larger initial size. Droplets with the same initial size can, however, possess different dissolution rates. We ascribe the non-universal dissolution rates to chemical and geometric surface heterogeneities (that lead to contact line pinning) and cooperative effects from the mass exchange among neighbouring droplets.

We experimentally investigate the splashing mechanism of a millimeter-sized ethanol drop impinging on a structured solid surface, comprised of micro-pillars, through side-view and top-view high speed imaging. By increasing the impact velocity we can tune the impact outcome from a gentle deposition to a violent splash, at which tiny droplets are emitted as the liquid sheet spreads laterally. We measure the splashing threshold for different micropatterns and find that the arrangement of the pillars significantly affects the splashing outcome. In particular, directional splashing in direction in which air flow through pattern is possible. Our top-view observations of impact dynamics reveal that an trapped air is responsible for the splashing. Indeed by lowering the pressure of the surrounding air we show that we can suppress the splashing in the explored parameter regime.

We investigate the existence of multiple turbulent states in highly turbulent TaylorCouette flow in the range of Ta = 10 11 to 9 × 10 12 by measuring the global torques and the local velocities while probing the phase space spanned by the rotation rates of the inner and outer cylinders. The multiple states are found to be very robust and are expected to persist beyond Ta = 10 13 . The rotation ratio is the parameter that most strongly controls the transitions between the flow states; the transitional values only weakly depend on the Taylor number. However, complex paths in the phase space are necessary to unlock the full region of multiple states. By mapping the flow structures for various rotation ratios in a Taylor-Couette setup with an equal radius ratio but a larger aspect ratio than before, multiple states are again observed. Here they are characterized by even richer roll structure phenomena, including an antisymmetrical roll state.