Whipping the feet is really serious business. Because they are used so much they are very slow to heal. If you want refined foot torture focus on the sensitive parts that don't directly touch ground. Between the toes or maybe the arch. Have encountered a few stories where slaves/prisoners feet were laced with thin leather straps with studs/pin pricks on the inside and bound so as to criss-cross these areas. Not sharp enough to break the skin deep, but to sting/irritate. The lady was made to walk/stand in these.