hallo! cau tra loi minh nhan dc tu mot native speaker ne`, share cho ban tham khao: "I don't sense much difference between them. 'expert in' suggests 'in some field of study'. 'expert on' suggests 'on some topic'. Many of the nouns that could follow can be taken either as a field of study or as a topic, so in many cases - maybe most cases - you can use either one"