Bulk flow investigation of solutal Marangoni flow

Depositing isopropanol (IPA) drops in the microliter range on the surface of a deep water layer induces a Marangoni flow as a result of the surface tension difference between IPA and water. The IPA sits on the water surface forming a quasi-static lens. From the contact line of the lens, an IPA film spreads in a radial direction on the surface until it eventually retracts. During this spreading and retracting process, the IPA lens dissolves into the water completely. All this happens in a duration on the order of a second. While the spreading process is studied quite extensively, including models of the spreading speed and spreading extent, the flow that is induced in the water has so far only been studied qualitatively, which also applies for the retraction process. Using Particle Image Velocimetry (PIV), we investigated the bulk flow of such a solutal Marangoni flow including the formation and the development of the tip vortex that appears due to the IPA film spreading on the surface. In addition to the qualitative description of the flow pattern, we can provide quantitative results of the flow field as a function of the drop volumes. Up to now it remains not entirely clear why the retraction process of the IPA film appears and it is also unclear what the time-scale of its appearance is. With the bulk flow analysis we can provide some further insights on the mechanisms leading to the retraction of the IPA film, including the quantification of the amount of fresh water that is transported towards the IPA lens at the surface. The fresh water transported to the surface may explain why the Marangoni flow is reversed, since the fresh water leads to a reversal of the surface tension gradient as compared to the beginning of the process.