The Marangoni effect is the mass transfer along an interface between two fluids due to the surface tension gradient (which fluid from areas with low surface tension is transferred to areas with higher surface tension).
In particular, a negative value of dγ/dT (Surface tension-temperature gradient), in which indicates that the surface tension (γ) reduces with an increasing temperature, prompts a radial outward flow of the molten metal and hence produces a wider and shallower melt-pool –> Less-penetration.
Conversely, a positive surface tension gradient (Temperature increased, surface tension also increased) leads to a radial inward flow; hence leading to a deeper and narrower melt-pool –> More penetration.
For most steels the Marangoni effect will be controlled by the Sulphur content, since the temperature coefficient of surface tension is negative when S < 30 ppm and this leads to a radially outward flow and poor-penetration, whereas a steel with S > 60 ppm has a positive (dγ/dT) which produces a radially inward flow giving deep-penetration (with narrow-weld-pool which may not allow gas-escaped that leading to porosity).
Addition to this topic is about Marangoni force ( or Thermocapillary forces) when welding of two different Sulphur content material (dissimilar welding or mis-match composition percentage) which play a part in the problems of ‘off–centre welding’, ‘porosity’ and ‘arc wander’ in GTAW welding in the surface rippling of welds.
Experiments have shown that during welding of two SS plates with different sulfur contents, the point of maximum penetration shifts towards the plate with lower sulfur content.