Fig. 4 shows the typical results for attachment efficiency (number of successful attachments in 10 observations) as a function of contact time. The attachment time is defined as the contact time at 100% attachment efficiency. As seen in Fig. 4, there is the critical contact time above which attachment efficiency increases dramatically. The reason for this behaviour is because above the critical contact time there is more time for drainage of liquid film between a bubble and a particle, resulting in stronger bubble–particle attaching force. Determination of this force as a function of contact time was beyond the scope of this work, and has not yet been quantified; however, it would be an interesting topic for future research.