Question: what is one condition for diffusion to take place
[{'answer': 'no - flux boundary', 'provenance': [{'wikipedia_id': '19908550', 'title': 'Diffusion', 'start_paragraph_id': 40, 'start_character': 32, 'end_paragraph_id': 40, 'end_character': 59, 'bleu_score': 1.0, 'section': 'Section::::Basic models of diffusion.:Diffusion flux.\n'}]}]