Small molecule propulsion driven by solvent-mediated ultrafast energy transfer from vibrations to self-translation

Despite its interest, self-propulsion at the molecular scale remains an open challenge. In this work, we demonstrate through all-atom molecular dynamics simulations that vibrational excitation of the amphiphilic molecule nitromethane ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msub> <a:mi>CH</a:mi> <a:mn>3</a:mn> </a:msub> <a:msub> <a:mi>NO</a:mi> <a:mn>2</a:mn> </a:msub> </a:mrow> </a:math> ) in water can lead to transient, directed propulsion. The excess vibrational energy is anisotropically transferred into the solvent, generating a mechanical instability that results in a short-lived net force between the solvent and the molecule. As a consequence, propulsion emerges along the axis of the molecule with speeds of up to <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:mn>2.2</b:mn> <b:mspace width="0.222222em"/> <b:mtext>Å</b:mtext> <b:mo>/</b:mo> <b:mi>ps</b:mi> </b:mrow> </b:math> , which decay in subpicosecond timescales. Under periodic excitation, this effect is measurable as an enhancement of the translational diffusion coefficient. We show that, despite being sensitive to the solute-solvent interaction model, the propulsion mechanism introduced here is robust to changes in the force field employed to describe atomic interactions. Our findings highlight a propulsion mechanism that is solvent-mediated and externally tunable via vibrational excitation, providing insight into the principles governing active transport at the nanoscale.