The white pixels in the original animation have been treated as solid cells (drawn in black), with a constant outwards force applied along the outline. Smoke is also gradually added in along the outline.
My other bad apple (using a particle-based fluid): https://www.youtube.com/watch?v=2Ni13dnAbSA
Support my random experiments on Patreon: https://www.patreon.com/SebastianLague
Credits: Music: https://www.youtube.com/watch?v=i41KoE0iMYU Animation by あにら (Anira). Specifically, I used this version: https://www.youtube.com/watch?v=FtutLA63Cp8 Subtitles: https://archive.org/details/bad-apple-resources