Invited talk by M. Rey at the University of Granada, 01 December, 2022

Drawing of a coffee mug using only coffee. (Image by M. Rey.)
Marcel Rey got invited to present his recent work on stimuli-responsive emulsions and the coffee ring effect at in the group seminar of the Laboratory of Surface and Interface at the University of Granada.

In the seminar, Marcel Rey talked about his recent advances on understanding the behaviour of stimuli-responsive emulsions and afterwards introduced a simple yet versatile strategy to overcome the coffee ring effect and obtain homogeneous drying of particle dispersions.

Temperature-responsive emulsions combine the long-term stability with controlled on-demand release of the encapsulated liquid. The destabilization has previously been attributed to microgel shrinkage, leading to a lower surface coverage which induces coalescence. We demonstrated that breaking mechanism is fundamentally different than previously thought. Breaking only occurs if the stabilizing soft microgel particles assume a characteristic double-corona microstructure, which serve as weak link enabling stimuli-responsive emulsion behavior. Conversely, emulsions stabilized by regular single-corona microgels remain remarkably insensitive to temperature.

After spilling coffee, a tell-tale circular stain is left by the drying droplet. This universal phenomenon, known as the “coffee ring effect”, is observed independent of the suspended material. We recently developed a simple yet versatile strategy to achieve homogeneous drying of dispersed particles. Modifying the particle surface with surface-active polymers provides enhanced steric stabilization and facilitates adsorption to the liquid/air interface which, after drying, leads to uniform particle deposition. This method is independent of particle size and shape and applicable to a variety of commercial pigment particles promising applications in daily life.

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