The removal of droplets on surfaces by an (air-) flow is relevant, e.g., for cleaning processes or to prevent corrosion or damage of electronic devices. Still the condition for droplet movement is not fully understood. Droplets start to move downstream at a critical (air-) flow velocity vcrit. For increasing flow velocity, this process is related to a strong oscillation of the droplet. This oscillation is supposed to be a key mechanism for the onset of droplet movement in conjunction with the flow field around the droplet. We report on measurements in the wake of the adhering droplet by means of laser-Doppler velocity profile sensor and hot wire anemometry. Thanks to the excellent spatial and temporal resolution of laser-Doppler velocity profile sensor and its capability to measure bidirectional flows, a backflow region can be detected in the wake of the droplet. Therefore, it can be concluded that this backflow structure is the driving mechanism for the strong flow movement inside the droplet against channel flow direction found in previous work. Analyzing the frequency spectra of the flow velocity, it was found that the flow is also oscillating; frequency peaks are in the same range as for the contour oscillation. Based on frequency, diameter and flow velocity, a Strouhal number can be calculated. This Strouhal number is almost constant in the investigated regime of droplet volumes and is between 0.015 and 0.03. Therefore, it can be assumed that an aeroelastic self-excitation effect may be present that eventually leads to droplet movement. Graphic abstract

This paper reports on the hydrodynamic droplet instabilities of different sizes and viscosities due to shear forces in a rectangular channel. Water‐glycerine droplets of different volumes are investigated. A new and nonambiguous definition for the critical velocity of droplet detachment and a new mathematical correlation between the critical velocity vcrit and the fluid properties are presented. The measurements show that vcrit decreases with the droplet volume but at the same time the contour deformation increases. With increasing viscosity of the liquid droplet, i.e., higher glycerine mass fraction, the contour deformation becomes more prominent and an increase in vcrit can be observed. With respect to the fluid properties and droplet volumes, three different motion patterns are detected.

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