Micro-Raman investigations on the structures of the surface and the inner of MgSO₄ droplets

Spherical MgSO₄ droplets were deposited by a syringe on the hydrophobic Teflon substrate. Using micro-Raman technique, the laser beam was highly focused twice on the surface and in the center of spherical droplets. The Raman spectra for the surface and the inner of MgSO₄ droplets were accordingly obtained, suggesting formation of a thin layer of gels on MgSO ₄ droplets at low relative humidity. The gel layer covered the surface and exhibited a significant delay in response to the change of ambient relative humidity, resulting in the structural difference between the surface and the inner of MgSO₄ droplets.