import math


def bond_number(r_max, sigma, rho_l, g):
    """ calculates the Bond number for the largest droplet according to
    Cha, H.; Vahabi, H.; Wu, A.; Chavan, S.; Kim, M.-K.; Sett, S.; Bosch, S. A.; Wang, W.; Kota, A. K.; Miljkovic, N.
    Dropwise Condensation on Solid Hydrophilic Surfaces. Science Advances 2020, 6 (2), eaax0746.
    https://doi.org/10.1126/sciadv.aax0746"""
    l_y = math.sqrt(sigma / (rho_l*g))
    bond = r_max**2 / l_y**2
    return bond