Diamond is known for being a radiation hard material that is chemically inert in a wide temperature interval, which makes it an attractive option for use in high-temperature, corrosive environments, such as molten salt reactors. This study presents the first experimental investigation into the capillary-driven wicking behavior of deionized water and molten FLiNaK salt in open microfluidic troughs (MFTs) etched into type IIa diamond substrates. We demonstrate that spontaneous capillary flow (SCF) can occur on the oxidized diamond surface in MFTs, and that the wicking behavior of diamond is significantly influenced by channel geometry (with hydraulic diameters ranging from 6.8 to 47 µm) and surface wettability.

Using a generalized Washburn model in conjunction with the Cassie angle, we found that capillary flow rates increased as the hydraulic diameter increased. Specifically, the maximum wicking rate in a 47-µm channel reached 5.87 mm·s−0.5 for deionized water at room temperature (25°C), while for molten FLiNaK, the highest wicking rate was 1.26 mm·s−0.5 at a temperature of 560°C. We did not observe SCF for molten FLiNaK in channels with hydraulic diameters below 14 µm unless the channel depth was increased.

The theoretical predictions agreed well with the experimental results, validating the model’s applicability to both liquid water and high-temperature molten salts. This research provides a quantitative foundation for the design of diamond-based microfluidic alpha spectrometers for in situ monitoring of radioactive molten salts, and highlights the critical importance of surface engineering in achieving reliable molten salt delivery through capillary action.

Research highlights include the following:

1. The wicking behavior of deionized water and liquid FLiNaK in open microchannels is experimentally investigated.

2. Both generalized Cassie Law and Washburn’s equation are capable of predicting SCF of the deionized water and liquid FLiNaK.

3. The capillary-driven wicking of the test liquids is dominated by the surface wettability and the channel configurations, such as the dimension and the cross-sectional profile.

4. The salt-philicity of the diamond must be augmented to trigger SCF with the liquid FLiNaK for the channels with a hydraulic diameter below 14 µm.