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Rapid switching of interfacial tension with a pH trigger using peptide surfactant Lac21E. A bubble formed in a solution of Lac21E at pH 3 reached a low interfacial tension, which was rapidly increased on addition of NaOH to increase the pH to 7.4, but could be decreased by addition of HCl to restore the pH to 3 (arrows). The cycle could be repeated giving similar results. Inset: bubble images at pH 7.4 (top) and pH 3 (bottom).
