Background

The contact and frictional response of a hydrogel is dependent on the polymer structure at the gel surface. Recent work has shown that different mold materials in contact with the gel during polymerization will affect the resulting polymer density.

Objective

The tribological response of a gel with a ‘brushy’ less-dense polymer surface has not been thoroughly studied. Our goal was to perform a suite of tribological experiments to better understand the response of the less-dense layer.

Methods

In this work, we conducted indentation, creep, and sliding experiments with various loads, speeds, and probe materials to determine the impact of the less-dense layer on the contact and frictional behavior of polyacrylamide hydrogels. We additionally used micro-fluorescent particle exclusion to measure the contact areas throughout each experiment.

Results

Indentation revealed a non-Hertzian regime for the first 13–29 µm after first contact that has a weaker force response for a given indentation depth. Creep experiments showed that the surface layer relaxes poroelastically, with water exudation occurring within the gradient layer despite the low contact pressures. Friction was highly speed-dependent, with faster sliding speeds decreasing friction to values as low as 0.01; transient behavior was not seen for most of the experiments, suggesting that the surface layer is capable of quick water re-uptake when out of contact.

Conclusions

We have provided a deeper understanding of the contact and frictional response of this gradient-density surface layer, which will prove useful for hydrogel designs requiring ultra-low friction in a dynamic application.

中文翻译：

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梯度密度聚丙烯酰胺水凝胶表面的摩擦学表征

背景

水凝胶的接触和摩擦响应取决于凝胶表面的聚合物结构。最近的工作表明，聚合过程中与凝胶接触的不同模具材料会影响所得的聚合物密度。

客观的

尚未彻底研究具有“毛刷”的低密度聚合物表面的凝胶的摩擦学响应。我们的目标是进行一套摩擦学实验，以更好地了解密度较低的层的响应。

方法

在这项工作中，我们进行了各种载荷，速度和探针材料的压痕，蠕变和滑动实验，以确定低密度层对聚丙烯酰胺水凝胶的接触和摩擦行为的影响。在每个实验中，我们还使用微荧光颗粒排除法来测量接触面积。

结果

压痕揭示了在第一次接触后的前13–29 µm的非赫兹区，对于给定的压痕深度，其力响应较弱。蠕变实验表明，尽管接触压力低，但表层的孔隙弹性松弛，在梯度层内仍发生水渗出。摩擦力与速度密切相关，更快的滑动速度将摩擦力减小至0.01；在大多数实验中都没有观察到瞬态行为，这表明表面层在不接触时能够快速吸收水分。

结论

我们已经对该梯度密度表面层的接触和摩擦响应有了更深入的了解，这将证明对于在动态应用中要求超低摩擦的水凝胶设计很有用。