From single microgels to dense microgel monolayers - investigation by atomic force microscopy

Due to their unique properties, microgels have garnered large interest in recent times for various applications, including interfacial applications. In this work, we study the internal structure of microgels within microgel monolayers at solid-liquid interfaces using atomic force microscopy (AFM). To capture the rich phase behavior of microgels, the monolayers were deposited at different surface pressures, offering insights into their structural responses under varying compression conditions. The results of the measurements show that the confinement of microgels within a dense monolayer impacts their internal structure and leads to an increase in the contact stiffness, and therefore polymer density, from the third compression regime onward. Additionally, the compression of microgels at the solid-liquid interface leads to the emergence of an attractive interaction between microgels and the sharp AFM tip, resulting in jumps-to-contact in the recorded force-distance curves. The occurrence of jumps-to-contact at high lateral compressions suggests a structural transition within the microgel monolayer, leading to an increase in van-der-Waals and/or electrostatic interactions between microgel and AFM tip.