def shrink(monolayer, cell, shrink_rate):
    """Divides the equilibrium volume of the cell
    by a factor (1+shrink_rate) and its equilibrium area
    by (1+shrink_rate)^2/3
    """
    factor = 1 + shrink_rate
    monolayer.cell_df.loc[cell, 'prefered_vol'] /= factor
    monolayer.cell_df.loc[cell, 'prefered_area'] /= factor ** (2 / 3)
