Many equations and correlations used in the design and optimization of fluidized unit operations are based on the assumptions particles that interact only with each other are collisional. However, highspeed video and image analysis suggest that in many cases particle-particle interactions go beyond Newtonian physics. Particles were found to have a strong propensity to cluster, especially fines. Result shows that particle clustering is prolific in many fluidizing environments from bubbling fluidized beds to riser to jets. For many empirical correlations, this is not an issue and cohesive behavior is often captured in the fitting of the experimental data. This is not the case for fundamental models where first principles rarely include cohesive forces. If they are included it is typically with non-mechanistic models as the nature of these cohesive forces are not completely understood. This work provides an insight to the nature of particle clustering in bubbling fluidized beds, risers and liquid jets. Each case appears to be due to a set of different cohesive forces or a sequence of cohesive forces. The nature of these forces is discussed. (C) 2013 Elsevier Ltd. All rights reserved.