Wei Zhang - Surfactant-Mediated Control of Colloid Pattern Assembly and Attachment Strength in Evaporating Droplets

Document created by Wei Zhang on Aug 22, 2014
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  Publication Details (including relevant citation   information): Morales, Verónica L., Parlange, Jean-Yves,   Wu, Mingming, Pérez-Reche, Francisco J., Zhang, Wei, Sang,   Wenjing, Steenhuis, Tammo S., Langmuir,   2013, 29 (6), pp 1831-1840

  Abstract: This study demonstrates that the   pattern assembly and attachment strength of colloids in an   evaporating sessile droplet resting on a smooth substrate can be   controlled by adding nonionic solutes (surfactant) to the   solution. As expected, increasing the surfactant concentration   leads to a decrease in initial surface tension of the drop, σ0.   For the range of initial surface tensions investigated (39?72 mN   m?1), three distinct deposition patterns were produced: amorphous   stains (σ0 = 63?72 mN m?1), coffee-ring stains (σ0 = 48?53 mN   m?1), and concentric rings (σ0 = 39?45 mN m?1). A   flow-displacement system was used to measure the attachment   strength of the dried colloids. Characteristic drying regimes   associated with the three unique pattern formations are   attributed to abrupt transitions of contact line dynamics during   evaporation. The first transition from slipping- to   pinned-contact line was found to be a direct result of the   competition between mechanical instability of the droplet and the   friction generated by pinned colloids at the contact line. The   second transition from pinned- to   recurrent-stick-rip-slip-contact line was caused by repeated   liquid film rupturing from evaporation-intensified surfactant   concentration. Data from flow-displacement tests indicate that   attachment strength of dried particles is strongest for amorphous   stains (lowest surfactant concentration) and weakest for   concentric rings (highest surfactant concentration). The   mechanism behind these observations was ascribed to the formation   and adsorption of micelles onto colloid and substrate surfaces as   the droplet solution evaporates. The range of attachment forces   observed between the colloids and the solid substrate were well   captured by extended-DLVO interactions accounting for van der   Waals attraction, electric double layer repulsion, and   micelle-protrusion repulsion. Both empirical and theoretical   results suggest that an increasingly dense layer of adsorbed   micellar-protrusions on colloid and substrate surfaces acts as a   physical barrier that hinders strong van der Waals attractive   interactions at close proximity. Thereby, colloid stains dried at   higher surfactant concentrations are more easily detached from   the substrate when dislodging forces are applied than stains   dried at lower surfactant concentrations.

  Address (URL): http://dx.doi.org/10.1021/la304685b