This due to the fact that the capillary potential  (curvature x surface free energy) much higher at the edges and the corners of the object, which enhances the edging process by local mass extraction plus the gradient of capillary potential induced the surface drift diffusion of the extracted  mass towards to the smooth regions of the surface.

Rigorous Variational general  treatment of this problem was presented in the following paper which was also selected to appear in Virtual Journal of  Nano Science and Technology, There it was shown that at  the critical points such as triple junction (where  only the left and right side curvatures are defined) the difference  between   the  left and right side bi-product  of the curvature versus surface free energy terms enters as a driving force for the mass transfer across the junction:

Driving Force =  Left (curvature x Surface Stiffness ) -Right (curvature x Surface Stiffness)   where in the case of anisotropy  not only the surface free energy  g  but also the second derivative of energy g''   would have discontinuity at the singular points such  triple junctions, edges and corners. 

NOTE: The following  function is also called Herring’s surface chemical potential denoted by  Mhu= k [ g’’ +g ]  in the literature. In our  formalism,  g+ =[g ‘’ +g]   is called the surface stiffness and k  is used to denote the local curvature (left and right) , which is assumed to be positive for a concave bulk surface.

Great thanks Dr. Oğurtani (Teşekkür ederim) and Dr. Chelgani (Mersi)

You are most well come Dear Dr. Hassan.  Regards

Be also aware of that etching can be anisotropic. That menas that certain crystaline phases are more soluble (etchable) than other in a certain etching liquid. Tyypical example is silicon that is etched by NaOH only at certain crystal direction. i think certain AFM tips of silicon (very sharp) are sharpend by anisotropic etching. It is good to Know this since small micro-nano structures may have a crustallization pattern different from the corresponding bulk materials.

Thank you Dr. Elwing;

I am working on "glass" as surface and HF as etchant. Since glass is amorphous then the surface of it is isotropic. do you think the dissolution of isotropic surface can also be anisotropic?

By definition - no.

But I suspekt that sio2 glass also can be anisotropic to some extent at the nano-scale depending on the method of fabrikation. Quartz made of sio2 is a crystal but I dont Know how HF etch the different crystal phases.