Critical capillary absorption of current-melted silver nanodroplets into multiwalled carbon nanotubes.

Since the discovery of carbon nanotubes (CNTs), [ 1 ] scientists have intensively investigated their intrinsic characteristics and tried to explore their properties after combining them with other materials. [ 2–15 ] Due to the large aspect ratio and a generically uniform diameter, the inner cavity of a CNT has been used as a nano test tube, siphon, catalyst carrier, and so on. [ 4–8 ] Recently, CNT composites made by fi lling the CNTs with various materials were found to enhance the CNTs’ electrical, thermal, optical, and mechanical properties; [ 9–15 ] thus, many methods have been developed to fabricate various CNT composites. For some CNTs grown with catalysts, such as Cu and Co 9 S 8 , the formation of composites is usually explained by capillary action. [ 16–18 ] Capillarity, by which the liquid is spontaneously absorbed into the narrow tube, is important for the control of channel-based devices in some biological and physical applications. The CNT is one of the smallest capillary tubes that is known on the nanoscale. Earlier studies suggest that only low-surface-tension materials can be introduced into CNTs by capillarity. This nanocapillarity has been held accountable for the various transition metals encapsulated inside the CNTs after CNT growth. [ 11 , 16 ] According to recent theoretical calculations and molecular dynamics simulations, the single-walled carbon nanotube (SWCNT) with an open end can act as a “capillary pipette”, which will also absorb nonwetting metallic nanodroplets. [ 19–21 ] Despite all these theoretical and post-factual fi ndings, no real-time process of capillary absorption of nonwetting metal droplets by CNTs has been observed. Herein, we report the real-time capillary action of liquid Ag nanodroplets produced by electriccurrent heating into the CNTs. Our observation reveals that the capillary absorption of molten Ag droplets can occur only