Sentence:  A <WSD>Janus</WSD> fluid is one made of <WSD>Janus</WSD> particles immersed in a solvent. A <WSD>Janus</WSD> particle like the Roman God Janus, ... is one that has two faces with two different functionalities. 
Meaning of the word marked with <WSD> tags:  (faces (such as animals with diprosopus) or aspects; or made of two different materials; or having a two-way action.)