Abstract
Although the interaction of automotive sprays with thin films is of high technical relevance for IC engine applications, fundamental knowledge about underlying physical mechanisms is still limited. This work presents a systematic study of the influence of the film’s initial thickness—homogeneously spread over a flat wall before the initial spray im**ement—on film surface structures and thickness after the interaction. For this purpose, interferometric film thickness measurements and complementary high-speed visualizations are used. By gradually increasing the initial film thickness on a micrometer scale, a shift from a regime of liquid deposition (increasing film thickness with respect to initial film thickness) to a regime of liquid removal (decreasing film thickness with respect to initial film thickness) is observed at the stagnation zone of the im**ing spray. This transition is accompanied by the formation of radially propagating surface waves, transporting liquid away from the stagnation zone. Wavelengths and amplitudes of the surface waves are increased with increasing initial film thickness.
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Notes
The only external study known to the authors, describing spray im**ement onto films of a defined, micrometer-sized thickness, is found in Ge et al. (2017). However, contrary to the diesel spray im**ement at hot and pressurized ambient conditions described in this study, the work of Ge et al. (2017) aimed for an investigation of different fuel blends at room conditions. Thus, their study is considered as not comparable to this study and is not discussed any further.
Spin coating is widely used in industry for generation of structures down to a nanometer scale, see e.g. Madou (2011).
Owing to the temperature sensitivity of the bearings and sealings, the spin-coating device in its current state is designed for a maximum ambient pressure of 10bar and a maximum ambient temperature of approximately 400K.
For a further description, see e.g. Figure 2 in Schnell et al. (1995).
The WLI’s minimum (\(\delta _\text {meas,min}\)) and maximum (\(\delta _\text {meas,max}\)) measurement range depends on the refractive index of the liquid (\(n_\text {d}\), Table 3) and is calculated as \(\delta _{\text {meas,min}} = \frac{3\,\upmu {\mathrm{{m}}}}{n_\text {d}}\) and \(\delta _{\text {meas,max}} = \frac{180\,\upmu {\mathrm{{m}}}}{n_\text {d}}\), respectively.
The film is illuminated diffusively from behind by a metal-halide lamp. Light encountering a highly inclined (thus unstable) film-air surface is refracted away from the lens (Hecht 2017) whose numerical aperture is limited. This consequently leads to a lower spatial intensity on the camera sensor and is interpreted as the wavefront. For areas with small wave inclination, light refraction is low and a higher intensity is observed. For a true quantitative information about the wavelengths, information on local film thickness distribution is needed. Gathering this information through (e.g.) high-resolving interferometry or planar laser-induced fluorescence is of considerably higher complexity.
Surface waves have been indicated for the 3-\(\upmu\)m case in Fig. 6a, as wave formation zone is very small for this case.
The dots represent average wavelengths and the horizontal lines inside the gray boxes correspond to median values. The bottom and top of the gray boxes are the first and third quartiles and thus, show the range of the mid 50% of the distribution. The outlying whiskers describe the entire width of the distribution.
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The authors kindly acknowledge financial support of this work by the German Research Foundation (KN 764/17-1).
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Seel, K., Reddemann, M.A. & Kneer, R. Optical investigation of the interaction of an automotive spray and thin films by utilization of a high-pressure spin coater. Exp Fluids 59, 50 (2018). https://doi.org/10.1007/s00348-018-2505-4
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DOI: https://doi.org/10.1007/s00348-018-2505-4