title: Effect of Poly(ethylene oxide) Molecular Weight on the Pinning and Pillar Formation of Evaporating Sessile Droplets: The Role of the Interface.
abstract: We report on the drying process of sessile droplets of aqueous poly(ethylene oxide) (PEO) solutions studied by contact angle analysis. Liquid samples were prepared with the same initial concentration of four different molecular weights, Mw, of PEO. Droplets with initial volumes of between 1 and 5 μL were left to evaporate while temperature, pressure, and relative humidity were kept constant. Residues were formed with either a disklike puddle or a distinctive tall conical pillar shape. The latter occurred following a four-stage deposition process: pinned drying, during which the contact line is stationary; pseudodewetting, where the receding contact line is induced by precipitation; bootstrap building, during which the liquid droplet is lifted on freshly precipitated solid; and late drying. Contact angle analysis allowed us to monitor all stages during drying and consider transitions between stages for different molecular weights. We illustrate the mechanisms taking place during the crucial stages of pinning and depinning, revealing the effect of adhesion and contact line friction for high molecular weights and its influence on the final morphology of the dried PEO solute. To this end, we performed PEO solution droplet evaporation on PEO and PTFE films demonstrating the importance of interfacial interaction phenomena. We show that the formation of disklike puddles for high molecular weights on glass is associated with continuous droplet contact line pinning. This results from the strong adhesion due to the interdigitation of the loops and tails of a polymer layer (adsorbed on glass during evaporation) with the polymer gel network inside the droplet that forms as water evaporates.
authors: †Institute for Materials and Processes, School of Engineering, The University of Edinburgh, King's Buildings, Edinburgh EH9 3FB, United Kingdom.
authors: Dimitrios
authors: D
authors: Mamalis
authors: †Institute for Materials and Processes, School of Engineering, The University of Edinburgh, King's Buildings, Edinburgh EH9 3FB, United Kingdom.
authors: Vasileios
authors: V
authors: Koutsos
authors: †Institute for Materials and Processes, School of Engineering, The University of Edinburgh, King's Buildings, Edinburgh EH9 3FB, United Kingdom.|⊥International Institute for Carbon-Neutral Energy Research (I2CNER) and Department of Mechanical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.
authors: Khellil
authors: K
authors: Sefiane
authors: †Institute for Materials and Processes, School of Engineering, The University of Edinburgh, King's Buildings, Edinburgh EH9 3FB, United Kingdom.
authors: Antonia
authors: A
authors: Kagkoura
authors: †Institute for Materials and Processes, School of Engineering, The University of Edinburgh, King's Buildings, Edinburgh EH9 3FB, United Kingdom.|§Impact Solutions, 16 Abbotsinch Road, Grangemouth FK3 9UX, United Kingdom.
authors: Michail
authors: M
authors: Kalloudis
authors: ‡Université de Bordeaux, I2M, UMR 5295, F-33400 Talence, France.|∥CNRS, I2M, UMR 5295, F-33400 Talence, France.|∇Arts et Metiers ParisTech, I2M, UMR 5295, F-33400 Talence, France.
authors: Martin E R
authors: ME
authors: Shanahan