Flow boiling in miniaturised channels is one of the most promising solutions for the efficient heat removal from next-generation high-power-density electronic devices. The existing models in design tools used to predict boiling heat transfer rates are largely based on empirical correlations and are very specific for the regime of boiling observed. A unified model that would be entirely based on the underlying physics of boiling and vapour bubble formation is an elusive task primarily due to our limited understanding of flow boiling. In this paper, we present results from an experimental investigation of the fundamental physics of a single vapour bubble formation inside a vertically oriented minichannel with a square cross-section and a hydraulic diameter of 5 mm. The experiments were performed for a selected set of mass fluxes (33-100 kg/m2s) and inlet subcooling degrees (5-15 K) using HFE-7100 as the working fluid. The test section is optically accessible from all directions, with one side of the channel fitted with an indium tin oxide (ITO) coated sapphire substrate to locally heat the fluid and generate vapour bubbles. Using diffuse backlight and a high-speed camera, visual observations of the vapour bubble formation process from a natural nucleation site for heat fluxes in the range 1.8-2.2 kW/m2 was conducted. The recorded time-lapse images were post-processed to extract the dynamic variations of vapour bubble features including width, height, base diameter, equivalent diameter, growth rate and sliding velocity over multiple bubble cycles. Using the least square circle fit method, the dynamic variations of upstream and downstream contact angles were also quantified. The temporal evolution of the bubble dynamic characteristics was compared to the isothermal bubble growth model in order to assess the conformity of their results, and gain insight into changes in the superheat level of the liquid phase adjacent to the growing vapour bubble.