The behavior of Pt-coated Janus particle in an aqueous solution of hydrogen peroxide is studied using atomic force microscopy (AFM). The catalytic chemical reaction between the platinum and hydrogen peroxide generates a propulsion force to propel the Janus particle in the liquid. To characterize the self-propelling nature of the Janus particle, we designed and assembled a new AFM probe with a Janus particle glued on the free end of the hanging fiber. The self-propulsion force generated by the Janus particle is then measured by recording and analyzing the capillary-force-versus-travel-distance curve and the power spectrum ∣z(w)∣² of the modified hanging fiber probe at an optimal immersion depth of the liquid-air interface. Meanwhile, theoretical attempts were made to determine th...[ Read more ]

The behavior of Pt-coated Janus particle in an aqueous solution of hydrogen peroxide is studied using atomic force microscopy (AFM). The catalytic chemical reaction between the platinum and hydrogen peroxide generates a propulsion force to propel the Janus particle in the liquid. To characterize the self-propelling nature of the Janus particle, we designed and assembled a new AFM probe with a Janus particle glued on the free end of the hanging fiber. The self-propulsion force generated by the Janus particle is then measured by recording and analyzing the capillary-force-versus-travel-distance curve and the power spectrum ∣z(w)∣² of the modified hanging fiber probe at an optimal immersion depth of the liquid-air interface. Meanwhile, theoretical attempts were made to determine the physical origin of the propulsion force. A new propulsion peak was found in the measured power spectrum ∣z(w)∣² in the low frequency range of 100 - 300 Hz. The concentration dependence of the propulsion peak location and height were analyzed to find the catalytic chemical reaction rates between Pt and hydrogen peroxide and the height fluctuation of the contact line near the Janus particle equator. The obtained values of the production rate k₁ of the intermediate product Pt(H₂O₂) and the production rate k₂ of the solute molecule O₂ for the catalytic reaction are found in reasonable agreement with those obtained from previous experiments by particle tracking. The fluctuating height h of the contact line is analyzed theoretically. Compared to the traditional method of particle tracking, the AFM measurement is faster and more accurate, and provides a new reliable way of determining the chemical reaction rate. The newly developed AFM method will become a useful tool for the study of dynamic properties of various micro- or nano-scale systems.