The 26th Heat Transfer Photogallery was sponsored by the K-22 Heat Transfer Visualization Committee for the 2018 International Mechanical Engineering Congress and Exhibition (IMECE) held in Pittsburgh, PA on Nov. 9–15, 2018. Nine entries were submitted into the Photogallery sessions, and the peer-reviewed evaluation conducted by both the participants and selected HTD K-22 Technical Committee members has identified the five final entries for publication in this ASME Journal of Heat Transfer October issue of 2019.

The purpose of publishing these entries is to draw attention to the innovative features of visualization techniques and esthetic qualities of heat and mass transport phenomena. This Photogallery issue covers multiscale visualizations of gas dynamics, mass-transfer, droplet coalescence, condensation and boiling heat transfer on nanoengineered surfaces, and droplet–surface interactions. To focus on visualization images and schematics for each entry, the text is kept to a minimum and further details should be found directly from the authors. Our wish is that the journal readers enjoy viewing these collections, acquire knowledge of the state-of-the-art visualization techniques, and also promote their participation in future Photogallery sessions in the ASME-SHTC and ASME-IMECE conferences.