After earning her degree, she moved to Unilever in Vlaardingen, where she joined the Physical Chemistry group led by Max van den Tempel.

Recognising her talents, Van den Tempel encouraged her to work towards a PhD, and in 1962 Lucassen-Reynders defended her dissertation on ‘Stabilization of water in oil emulsions by solid particles’ at Utrecht University in front of Theodoor Overbeek[1] Lucassen-Reynders' early research considered the impact of fat crystals on the stability of water in emulsions with oils.

Based on the Pickering mechanism, the fat crystals would stabilize this emulsion, in which solid particles are wetted by the continuous phase.

Lucassen-Reynders used Young's equation and Gibbs' law to explain the relationship between the interfacial tensions and adsorptions of the solid, oil and water.

[1] The consideration of ionic surfactants introduced compilation due to their surface electric potentials, which didn't match the neutral requirements of Gibbs' adsorption law.

Also a physical scientist, Lucassen’s work, including many collaborations with Lucassen-Reynders, is well recognised in an Honorary Note [Advances in Colloid and Interface Science 107 (2004) 1–7 ] and in his obituary.

Amsterdam: Elsevier Science B.V.; 1998. p. 341. behaviour of proteins at water/fluid interfaces in the framework of a two- dimensional solution model.