不止好看！化学所宋延林研究员Angew赏析——基于可调模板退浸润控制颗粒的尺寸分选组装图案

他们利用硅柱微模板制备了两种和三种颗粒的尺寸分选组装图案，并分析研究了颗粒的组装过程和组装机理，探讨了对不同组装结构的控制因素。不同于以前的工作只能实现线形非闭合的组装图案，在此研究中通过构建环绕硅柱的液桥，实现环状的组装图案。结合理论模型计算，深入研究液体在模板上的退浸润对颗粒组装的影响。研究结果发表在Angew. Chem. Int. Ed. 杂志（Angew. Chem. Int. Ed. 2018, 57, 1-6）。

在超亲水基底上，由于外向的毛细流和气-固-液三相接触线附近界面的楔形限域条件下，大小颗粒在三相接触线附近有了尺寸分离：小颗粒更接近三相线而大颗粒相对远离三相线。在退浸润的过程中，三相线推动保持有大小分离状态的颗粒从模板边缘向模板的中间位置运动。在气液界面遇到硅柱表面时，液体和颗粒发生了如Figure 1b中的变化，最终得到了环状的组装结构。

Figure 1. Formation of the dual-ring patterns and the structural diversification. a) Mixed two-sized particle suspension is sandwiched between a pillar template and a superhydrophilic cover. b) The time-sequenced snapshots show the binary particle assembly process during the suspension dewetting the superhydrophilic pillar template, schematically illustrated by the inserted sketch. The white arrows indicate the time sequence, and blue arrows indicate the inner capillary flow. c) The laser scanning confocal microscope image and the inserted SEM image show the dual-ring assembly patterns from binary particles with diameters of 1 µm and 0.37 µm. d) Assemblies obtained by increasing Vf_L/S   to 3 (top image; decreasing the volume fraction of small particles from 0.2% to 0.1%) or decreasing D_L/S  to 2.13 (bottom image; increasing the diameter of the small particles from 0.37 µm to 0.47 µm).

研究发现利用不同凸边形的模板，能够实现不同的组装图案，如Figure 2所示, 最终的结果都是大颗粒围绕硅柱边缘，小颗粒围绕大颗粒外侧（远离硅柱）的边缘。这样的尺寸分离是由于在亲水基底上，三相线附近的楔形限域空间决定的。对于三种颗粒的组装，也会得到颗粒由大到小逐渐径向远离硅柱的尺寸分选组装图案，但是最小颗粒容易在较大尺寸的颗粒组装间隙分布，这是受到颗粒填充原理的影响。通过改变硅柱的高度，调节液体软模板的限域空间，能够对于不同尺寸的颗粒组装进行有效的控制。能够发生限域组装的颗粒尺寸随着硅柱高度的增加而增大。

在后退角大于45°的疏水基底上，颗粒的组装又会出现不同的形式。首先，液体的边缘界面形状在疏水基底上发生了改变，在气液界面开始接触到硅柱表面时，如Figure 3所示, 三相接触线附近的气液界面与硅柱表面组成的限域空间不足以满足大颗粒的进入，只有小尺寸颗粒能够在硅柱前端被限域，而大颗粒被阻挡在硅柱的后面，这样就对大小颗粒的限域组装进行了选择。不同于以前的只在亲水基底上能够实现大小颗粒的分离，这里在疏水基底上实现了大小颗粒的分离，并能够控制其组装图案。通过实验和理论分析，能够通过简易的实验操作控制液体退浸润的方向。对于截面具有各向异性形状的硅柱模板，通过调节液体退浸润的方向，最终得到不同的组装图案。

Figure 2. Control of the dual-ring assembly, formation of different-shaped patterns, and ternary particle assembly. a) Schematic illustration of two-sized particles in the capillary confinement upon the air-liquid interface becoming discontinuous. b) The dependence of the theoretical maximum diameters of the large particles D_m (the black solid line) and the experimental diameters of the large particles D (the scattered dots) on the pillar height H, illustrating that both D_m and D show a positive correlation with H and that D is smaller than D_m. c) The confocal micrographs and SEM images show the dual-ring assemblies in triangular and pentagonal patterns. d) Schematic illustration of ternary particles in the capillary confinement near the pillar, confocal micrograph of the ternary-ring assembly array and the inserted SEM image, showing that the largest particles of 1 μm constitute the innermost ring, the smallest particles of 0.18 μm constitute the outermost ring, and the medium-sized particles of 0.37 μm mixed with the smallest particles constitute the middle ring.

Figure 3. The formation of patterns with component separation. a) The time-sequenced snapshots show the binary particle separation process on the pillar template with a receding contact angle larger than 45°. The white arrows indicate the time sequence. b, c) The top- and side-view illustration of the confined assembly of the large and small particles at the front and rear region of the pillar, respectively. The blue arrows indicate the capillary force on the confined particles. d) The confocal micrographs and the inserted magnified SEM images show different patterns with the separation of 1 µm and 0.37 µm particles. The yellow arrows indicate the dewetting direction.