Henna has a tannin dye molecule, Lawsone, or hennotannic acid, which is small enough to penetrate a skin cell. If you put henna paste on skin, the dye molecules will penetrate down the columns of skin cells. They don’t spread out, as ink would on blotter paper, they go straight down as ink would on corrugated cardboard. That is why the pattern stays clear and in place till the last day of exfoliation.
The skin cells closest to the henna paste will have the greatest dye saturation. The skin cells farthest from the henna paste will have the least dye saturation.
Very thin stratum corneum will never take henna stain as well as very thick stratum corneum. In thin areas of stratum cornum, cells go quickly from being living, flexible skin cells to shedding from the surface as dead cells. These cells do not have time to become corneated. Your eyelid has a very thin layer of stratum corneum, and that skin is soft and flexible. Your heel has a very thick layer of stratum corneum and that area becomes hard and dry. Your heels may be highly corneated. Henna does not stain thin, moist, living tissue well. It stains hard, thick, dry, dead, corneated cells. Therefore, henna will never stain dark or last long on areas of thin stratum corneum.