Janus微纳马达是具有不对称物理结构或化学性质的微纳尺度的颗粒，可以通过其所处环境中的物理或化学反应，将化学能等能量转化为自身运动所需要的机械能，从而完成自驱动运动。当Janus微纳马达在周围局部流体中建立浓度场梯度来实现运动时，其运动被称为自扩散泳动。Janus微纳马达在生物医学中作为药物输运的载体或在复杂工况中作为微纳机器人的动力部件展现出了广阔的应用前景。因此，其运动机理——自扩散泳——的研究也是当前学术界的热点。

   已有研究对Janus微纳马达在水溶液等简单流体中的运动已取得了重要的进展。然而，实际应用中，Janus微纳马达工作的流体介质往往是生物黏液或高聚物等复杂流体。对微纳马达在复杂流体中的运动机理及特性的研究仍非常缺乏。这些复杂流体往往具有复杂的微观结构及非线性的本构关系，自主运动的Janus微纳马达在复杂流体中的运动产生丰富的流动现象，引入了新颖的物理机理，是微纳米流体力学与软物质物理研究交叉的前沿问题。

   对此，本文通过实验测量了直径2mm的Pt-SiO₂型Janus球形微马达在高聚物聚氧化乙烯（PEO，分子量M_w=10⁵）和过氧化氢（H₂O₂）的混合溶液中自扩散泳动的运动特性。Janus微马达表面的Pt催化溶液中的H₂O₂分解为水和氧气，使镀Pt侧附近氧分子浓度升高，产生浓度梯度，进而形成Janus微马达向低浓度一侧运动的自扩散泳。混合溶液中PEO质量分数分别为0%、0.1%、0.5%、1.0%，H₂O₂浓度分别为10%和15%，通过调控溶液中PEO的浓度来调整高聚物溶液的微观网络结构尺寸及黏弹性。实验采用颗粒追踪方法，通过倒置荧光显微镜（Olympus IX71）观察，并由微球轨迹追踪法测量不同浓度的PEO与H₂O₂溶液中Janus微马达自扩散泳特征速度、均方位移（Mean Squared Displacement, MSD）、均方转角（Mean Squared Rotational Angle, MSRA）和位移概率分布（Displacement Probability Distribution，DPD）等基本结果。

   实验结果系统描述了溶液中PEO及H₂O₂浓度对Janus微马达自扩散泳平动运动的影响。实验发现Janus微球的平动自扩散泳速度随PEO浓度的增大而减小，随H₂O₂浓度的增加而增加。这是由于溶液中高聚物PEO浓度越高，混合溶液的黏度也越大，Janus微球在溶液中的运动需要克服更大的黏性阻力；溶液中H₂O₂浓度较高时，Janus微球Pt表面的的催化反应速率更快，为浓度梯度的建立及自扩散泳运动提供更加充分的“原料”和“动力”。

       对均方位移MSD和时间间隔t进行无量纲化，得到无量纲的均方位移<(Dr)²/d²>随无量纲时间t=t/t_r(其中t_r=1/D_r，D_r为旋转扩散系数)的变化，显示了Janus微球在高聚物溶液中自扩散泳的三阶段运动特征。首先，t < 0.03是短时间亚扩散段，幂次约0.8-1范围。之后，0.03 < t < 1是自驱动推进段，幂次在1.2-1.9范围，表现出超扩散特征，可以看到幂次随PEO浓度的增大而减小。最后是长时间段t >1时，无量纲均方位移MSD与无量纲时间t 的幂次回到接近于1。对时间间隔t的无量纲化基于旋转特征时间t_r，说明微球自身旋转历经各角度后会展现出类布朗运动的MSD ~ t变化特征。

    本文还系统测量了Janus微马达在高聚物溶液中自扩散泳动的转动特征。通过MSRA 随t的变化并与理论预测MSRA对比发现，转动有两个特征阶段：短时间段MSRA随t变化的斜率较大，表现出较大的旋转扩散系数，依据MSRA计算出有效旋转扩散系数为理论值的10倍以上；长时间段MSRA随t变化的斜率较小，测量得到的有效旋转扩散系数与理论值相符合。MSRA的两阶段特征意味着Janus微球在短时间间隔内的快速转动向长时间的慢速稳定转动的过渡。实验结果显示Janus微马达在短时间段的MSRA随PEO浓度的增大而增加，而与H₂O₂浓度无关。这可能源于微观高聚物长链与微球相互作用导致的转动脉动。长时间段旋转扩散系数取决于PEO黏度，与H₂O₂浓度无关。通过对转角的统计特性进行分析，发现在混合溶液中，PEO和H₂O₂浓度的增加都使得Janus微球在小转角处的概率略微下降。

    Janus micro/nano-motors is a kind of micro/nano scale particles with asymmetric physical structures or chemical properties. Through physical interaction or chemical reactions, the chemical energy and other energy from the environment can be converted into mechanical energy to support the self-propulsion of the Janus micro/nano-motors. When Janus micro/nano-motors establish a concentration gradient in the surrounding fluid, the self-propelled motion is called self-diffusiophoresis. Janus micro/nano-motors, as the carrier of drug transport in biomedicine or the power component of micro/nano-robot in complex working conditions, have shown a broad application prospect. Therefore, the research on the mechanism of self-diffusiophoresis, is also an important issue.

      The self-diffusiophoresis of Janus micromotor in simple fluid such as aqueous solution has been widely investigated. However, in practical application, the fluid medium of Janus micro/nano-motor is often complex fluid such as biological mucus or polymer. There is still a lack of research on the mechanism and characteristics of the Janus micro/nano-motor in complex fluid. These complex fluids often have complex microstructures and nonlinear constitutive relations. The self-diffusiophoretic Janus micro/nano-motor in the complex fluid produces rich flow phenomena. It introduces a novel physical mechanism, which is the frontier problem of the intersection of micro/nano hydrodynamics and soft matter physics.

      In this paper, the characteristics of the self-diffusiophoretic Pt-SiO₂ Janus micromotor with a diameter of 2 μm in the mixed solution of PEO (M_w=10⁵) and H₂O₂ were measured. The hydrogen peroxide decomposes into water and oxygen on the Platinum (Pt) surface of the Janus micromotor, which makes the concentration of oxygen molecules near the Pt side increase and generates a concentration gradient. The Janus micromotor will move to the low concentration direction due to self-diffusiophoreis in such a concentration field. The mass fraction of PEO in the mixed solution is 0%, 0.1%, 0.5%, 1.0% respectively, and the concentration of hydrogen peroxide is 10% and 15% respectively. By adjusting the concentration and molecular weight of PEO in the solution, the micro network size and viscoelasticity of the polymer solution can be adjusted. In the experiment, the characteristic velocity, the mean square displacement (MSD), the mean square rotation angle (MSRA) of Janus micromotor in PEO and H₂O₂ solutions of different concentrations were measured by particle tracking method.

    The experimental results systematically describe the effects of PEO and H₂O₂ concentration on the translational motion of Janus micromotor self diffusion swimming. It was found that the translational self diffusion velocity of Janus microspheres decreased with the increase of PEO concentration and increased with the increase of H₂O₂ concentration. Because the higher the PEO concentration is, the greater the viscosity of the mixed solution. When the concentration of H₂O₂ in the solution is increased, the catalytic reaction rate on the Pt surface of Janus microsphere is faster, which provides more "raw material" and "power" for the establishment of concentration gradient and self diffusion swimming.

       Dimensionless mean square displacement <(Dr)²/d²> and dimensionless time interval t = t/t_r were used to show that the three-stage characteristics of the self-diffusiophoresis in PEO solutions. First, t < 0.03 is a short-term sub diffusion regime in which the power of the MSD variation with time is about 0.8-1. After that, 0.03 < t < 1 is a propulsion regime, with the power in the range of 1.2-1.9, showing the characteristics of super-diffusion. It can be seen that the power decreases with the increase of PEO concentration. Finally, when t > 1 for a long period of time, the power of dimensionless mean azimuthal shift MSD and dimensionless time t, returns to close to 1. The dimensionless time interval t is based on the rotation characteristic time t_r, which indicates that the rotation of the microspheres through various angles will show the characteristics of Brownian like motion MSD ~ t.

       In addition, through the analysis of the statistical characteristics of the displacement, it is found that DPD has a peak distribution, gradually widens with the increase of time interval, and the peak value decreases, but the peak still exists in a long period of time.The influence of PEO concentration on DPD exists through the whole process. With the increase of H₂O₂ in the solution, the probability of DPD at large displacement increases, but it is affected by the concentration of PEO. The higher the PEO concentration, the greater the probability of DPD at small displacement, and the less obvious the effect of H₂O₂ concentration on DPD.

     The rotational characteristics of the self-diffusiophoretic Janus micromotor in polymer solution are also measured. According to the MSRA  ~ t relation, two stages of rotation are found: the short-term manifests a larger slope of the MSRA ~ t relation. The slop shows a larger rotation diffusion coefficient, and the effective rotation diffusion coefficient calculated by MSRA is more than 10 times of the theoretical value; the long-term is the linear regime with a smaller slope of the MSRA ~ t relation, and the effective rotation diffusion coefficient calculated by the slope is consistent with the theoretical value. The two-stage characteristic of MSRA means the transition from the fast rotation of Janus microspheres in a short time interval to the slow and stable rotation in a long time. The experimental results show that the MSRA of Janus micromotor increases with the increase of PEO concentration in a short period of time, but has nothing to do with H₂O₂ concentration. This can be attributed to the rotational pulsation caused by the interaction between the long chain of the micro polymer and the microsphere. The increase of PEO concentration in a long period of time makes the rotation angle increase, but the rotational diffusion coefficient depends on the viscosity of PEO and has nothing to do with the concentration of H₂O₂. By analyzing of the statistical characteristics of the rotational angle, it is found that the increase of PEO and H₂O₂ concentration in the mixed solution decreases the probability of Janus microspheres at smaller rotation angle.