Fig. 1

Force trap generated by two counterdirected thermoviscous flow fields that align with the direction of a particle’s displacement from the central stagnation point. a Experimental setup: rapid laser scanning (red beam) induces counterdirected flows (blue arrows) on a common axis (termed ‘compressional’ or ‘trapping’ axis) with a probe particle, giving rise to a stagnation point. Along each scan path, the direction of the flows runs opposite to that of the laser scanning. The blue arrows point along the compressional axis, also shown with horizontal white arrows in panel b. b Visualisation of the generated flow-field lines and its central stagnation point (purple) using tracer beads (Additional file 1: Video SV1), with horizontal and vertical arrows indicating the trapping and extensional axes, respectively. c Linear fit to the velocity-displacement plot of tracer particles near the stagnation point in a stationary flow field. The shaded lightblue region encloses data from particles in contact with the laser scan paths. d Snapshots demonstrating the restoring character of the thermoviscous flows (white arrows). Depending on the direction of particle displacement \(\delta r\) from the stagnation point, the flow field is automatically rotated (double-headed arrows) in a way that effectively counteracts this displacement (Additional file 2: Video SV2). Dotted lines indicate dynamically co-rotating trapping axis. e Rhodamine intensity profile (red arrows correspond to scan paths) and relative temperature increase upon application of the counterflows. The difference in the image intensity can be directly used to quantify the relative temperature increase as a function of laser driving current (LDC). The laser power we used throughout the experiments in this work corresponds to LDC of 900 mA and led to maximum heating along the scan paths of around 3 K, given that the intensity of the dye changes by 1.3%/K. The inset shows the level of heating in the region around the stagnation point at each LDC, where the micromanipulated object is trapped (maximum heating of 0.5 K). Error bars represent standard deviations, obtained from a total of 3 repeats for 700–800 mA and 2 repeats for 850–900 mA