The degree of wetting is quantitatively characterized by the dimensionless cosine of the contact angle (wetting angle). The quartz has a contact angle of zero degrees. Teflon is -108 degrees. The 90 degree cosine is zero.

Hi Xin

In the case of the droplet has relatively high surface tension, the maximum spread factor would be equal or less than 1, as Mr. Mirgorod has explained. In the case of the droplet has relatively low surface tension, the above is applicable, and the maximum spread factor could be more than 1.

Hi both,

Thanks a lot for your answer. I am not sure whether I have fully understood your explanation. But I assume that your point regarding the wetting of droplet on substrate is relevant to that the contact angle of the droplet with the substrate can determine the maximum spread ratio. When droplet has a low contact angle, the maximum spread ratio will be high and much larger than one. however, when the droplet has a high contact angle (>>90), the maximum spread ratio will be low and smaller than one.

But for my case, it is not the static wetting, but the maximum spreading during the droplet impaction on the substrate before the droplet reaches equilibrium state. So, from some previous papers, it was found that the maximum spread diameter is not strongly dependent with the contact angle, but more related to the Weber number (dependent with l-v surface tension) and Reynolds number. So, for this case, it is true that the maximum spread ratio will be always larger than one during the droplet impaction due to the deformation of the droplet?

Thanks again for the helps.

Xin

Hi Xi

Now, I do understand your question, as I mentioned earlier, the smaller the surface tension of the droplet the larger the maximum spread factor, more than 1, with respect to the impact of the droplet on a substrate.

Hi Khaled,

Thanks a lot for your answers. It helps a lot.

Xin

You are always welcome.

Hi Xi,

while wetting is a surface phenomenon, it would be better to study and to quantify it using surface and surface-energy parameters instead linear (diameters). I would suggest you to read a couple of papers (links below) in order to know the kappa parameter which caracterizes wetting from a thermodynamic standpoint. kappa is an addimensional ratio between surface areas of system consisting of a sessile droplet on a solid surface in a gas media:

k= (droplet_surface_area-before-wetting - droplet_air_interfacial_area_after_wetting)/interfacial_area_solid-liquid_created_by_wetting

http://www.nature.com/articles/s41526-017-0031-y

https://www.preprints.org/manuscript/201708.0062/v2

By complete wetting, kappa is minus one because the surface energy of the solid is so big, that all the liquid spreads on it forming a low energetic interface solid-liquid; and the liquid air interfacial area tends to grow over all the available solid surface, being equal to the interfacial solid-liquid area.

On the other hand, if the liquid does not wet the surface, kappa will tend to infinity, because the interfacial energy solid-liquid is so big, that no interface can be build by the system (the droplet remains spherical), ie. the interfacial area solid-liquid will be zero.

Regards,

Alfredo Calvimontes

Hi Alfredo,

Thanks a lot for your answer. It helps me better understand the physics behind the wetting. Very nice work.

Regards,

Xin

Hi Xin,

i uploaded yesterday a new version of the second paper at preprints:

https://www.preprints.org/manuscript/201708.0062/v3

This is by now under review at Microgravity Science and Technology (Springer).

Thank you for your feedback.

Regards,

Alfredo