Baik Seung Jae, Moldenaers Paula, Clasen Christian
Department of Chemical Engineering, Katholieke Universiteit Leuven, Willem de Croylaan 46, 3001 Leuven, Belgium.
Rev Sci Instrum. 2011 Mar;82(3):035121. doi: 10.1063/1.3571297.
A new generation of the "flexure-based microgap rheometer" (the N-FMR) has been developed which is also capable of measuring, in addition to the shear stress, the first normal stress difference of micrometer thin fluid films. This microgap rheometer with a translation system based on compound spring flexures measures the rheological properties of microliter samples of complex fluids confined in a plane couette configuration with gap distances of h = 1-400 μm up to shear rates of γ = 3000 s(-1). Feed back loop controlled precise positioning of the shearing surfaces with response times <1 ms enables to control the parallelism within 1.5 μrad and to maintain the gap distance within 20 nm. This precise gap control minimizes squeeze flow effects and allows therefore to measure the first normal stress difference N(1) of the thin film down to a micrometer gap distance, with a lower limit of N(1)/γ = 9.375×10(-11) η/h(2) that depends on the shear viscosity η and the squared inverse gap. Structural development of complex fluids in the confinement can be visualized by using a beam splitter on the shearing surface and a long working distance microscope. In summary, this new instrument allows to investigate the confinement dependent rheological and morphological evolution of micrometer thin films.
新一代的“基于挠曲的微间隙流变仪”（N-FMR）已被开发出来，它除了能够测量剪切应力外，还能够测量微米级薄液膜的第一法向应力差。这种具有基于复合弹簧挠曲的平移系统的微间隙流变仪，可测量以平面库埃特配置限制的微升复杂流体样品的流变特性，间隙距离h = 1 - 400μm，剪切速率高达γ = 3000 s⁻¹。反馈回路控制剪切表面的精确定位，响应时间<1 ms，能够将平行度控制在1.5 μrad以内，并将间隙距离保持在20 nm以内。这种精确的间隙控制可将挤压流动效应降至最低，因此能够测量薄膜的第一法向应力差N(1)，直至微米级间隙距离，其下限为N(1)/γ = 9.375×10⁻¹¹ η/h²，该下限取决于剪切粘度η和间隙平方的倒数。通过在剪切表面使用分束器和长工作距离显微镜，可以观察受限复杂流体的结构演变。总之，这种新仪器能够研究微米级薄膜的受限相关流变和形态演变。
