Wei Zhang - Colloid retention at the meniscus-wall contact line in an open microchannel

Version 1

      Publication Details (including relevant citation   information): Zevi, Yuniati, Gao, Bin, Zhang, Wei,   Morales, Verónica L., Ekrem Cakmak, M., Medrano, Evelyn A., Sang,   Wenjing, Steenhuis, Tammo S., Water Research,   2012, 46 (2), pp 295-306

      Abstract: Colloid retention mechanisms in   partially saturated porous media are currently being researched   with an array of visualization techniques. These visualization   techniques have refined our understanding of colloid movement and   retention at the pore scale beyond what can be obtained from   breakthrough experiments. One of the remaining questions is what   mechanisms are responsible for colloid immobilization at the   triple point where air, water, and soil grain meet. The objective   of this study was to investigate how colloids are transported to   the air-water-solid (AWS) contact line in an open triangular   microchannel, and then retained as a function of meniscus contact   angle with the wall and solution ionic strength. Colloid flow   path, meniscus shape and meniscus-wall contact angle, and colloid   retention at the AWS contact line were visualized and quantified   with a confocal microscope. Experimental results demonstrated   that colloid retention at the AWS contact line was significant   when the meniscus-wall contact angle was less than 16°, but was   minimal for the meniscus-wall contact angles exceeding 20°.   Tracking of individual colloids and computational hydrodynamic   simulation both revealed that for small contact angles (e.g.,   12.5°), counter flow and flow vortices formed near the AWS   contact line, but not for large contact angles (e.g., 28°). This   counter flow helped deliver the colloids to the wall surface just   below the contact line. In accordance with DLVO and hydrodynamic   torque calculations, colloid movement may be stopped when the   colloid reached the secondary minimum at the wall near the   contact line. However, contradictory to the prediction of the   torque analysis, colloid retention at the AWS contact line   decreased with increasing ionic strength for contact angles of   10–20°, indicating that the air-water interface was involved   through both counter flow and capillary force. We hypothesized   that capillary force pushed the colloid through the primary   energy barrier to the primary minimum to become immobilized, when   small fluctuations in water level stretched the meniscus over the   colloid. For large meniscus-wall contact angles counter flow was   not observed, resulting in less colloid retention, because a   smaller number of colloids were transported to the contact line.

      Address (URL): http://www.sciencedirect.com/science/article/pii/S0043135411005768