  Plasmonic nano-tweezers use intense electric field gradients to generate
optical forces able to trap nano-objects in liquids. However, part of the
incident light is absorbed into the metal, and a supplementary thermophoretic
force acting on the nano-object arises from the resulting temperature gradient.
Plasmonic nano-tweezers thus face the challenge of disentangling the intricate
contributions of the optical and thermophoretic forces. Here, we show that
commonly added surfactants can unexpectedly impact the trap performance by
acting on the thermophilic or thermophobic response of the nano-object. Using
different surfactants in double nanohole plasmonic trapping experiments, we
measure and compare the contributions of the thermophoretic and the optical
forces, evidencing a trap stiffness 20x higher using sodium dodecyl sulfate
(SDS) as compared to Triton X-100. This work uncovers an important mechanism in
plasmonic nano-tweezers and provides guidelines to control and optimize the
trap performance for different plasmonic designs.
