Molecular weight of 4-arm PEG in the fibril cluster also influence the emulsion stability. In our experiment, fibril cluster with a high PEG molecular weight (Mw: 2.5k  4 = 10 kDa) exhibits much stronger capability to stabilize dextran-in-PEG emulsion, compared to that with relatively low PEG molecular weight (Mw: 2 kDa and 5 kDa) (Fig. ). This observation can be explained by the Bancroft's rule that packing parameter of surfactants determines the emulsion type and stability. When fibril clusters are adsorbed at the W/W interface, the 4-arm PEG moiety has a high affinity to the PEG-rich continuous phase, while the fibril 'tails' tend to stay in the dextran-rich droplet phase. The volume ratios of the 4-arm PEG moiety to the fibril can be adjusted by changing the molecular weight of the PEG or their mass ratios. If the volume of the PEG moiety is larger than that of the fibril tail, the assembly of fibril cluster at W/W interface should maintain a positive curvature and better stabilize the dextran-in-PEG emulsion.