Puffing of a decane/ethanol emulsion droplet and a droplet group under convective heating and its effects on fuel/air mixing are investigated by direct numerical simulation that resolves all of the liquid/gas and liquid/liquid interfaces. With distinct differences in the boiling point between decane and ethanol, the embedded ethanol sub-droplets can be superheated and boil explosively. Puffing, i.e. ejection of ethanol vapour, occurs from inside the parent decane droplet, causing secondary breakup of the droplet. The ejected ethanol vapour mixes with the outer gas mixture composed of air and vapour of the primary fuel decane, and its effects on fuel/air mixing can be characterised by the scalar dissipation rates (SDRs). For the primary-fuel SDR, the cross-scalar diffusion due to ethanol vapour puffing plays a dominant role in enhancing the micromixing. When the vapour ejection direction is inclined towards the wake direction, the wake is elongated, but the shape of the stoichiometric mixture fraction isosurface is not changed much, indicating a limited effect on droplet grouping in a spray. On the other hand, when the ejection direction is inclined towards the transverse direction, the stoichiometric surface is pushed further away in the transverse direction, and its topology is changed by the puffing. The trajectories of ejected ethanol vapour pockets can be predicted by the correlation obtained for a jet in cross-flow, and the vapour pockets may reach a few diameters away from the droplet. Therefore, in a multiple-droplet configuration, the transverse ethanol vapour ejection due to puffing may transiently change the droplet grouping characteristics. In simulation cases with multiple droplets, the interaction changing the droplet grouping due to puffing has been confirmed, especially for droplets in the most upstream position in a spray. This implies that puffing should be accurately included in the mixing and combustion modelling of such a biofuel-blended diesel spray process.