Vélez-Cordero J Rodrigo, Velázquez-Benítez A M, Hernández-Cordero J
Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México , Apdo. Postal 70-360, México D.F. 04510, México.
Langmuir. 2014 May 13;30(18):5326-36. doi: 10.1021/la404221p. Epub 2014 Apr 30.
Thermocapillary flow has proven to be a good alternative to induce and control the motion of drops and bubbles in microchannels. Temperature gradients are usually established by implanting metallic heaters adjacent to the channel or by including a layer of photosensitive material capable of absorbing radiative energy. In this work we show that single drops can be pumped through capillaries coated with a photoresponsive composite (PDMS + carbon nanopowder) and irradiated with a light source via an optical fiber. Maximum droplet speeds achieved with this approach were found to be ~300 μm/s, and maximum displacements, around 120% of the droplet length. The heat generation capacity of the coatings was proven having either a complete coating over the capillary surface or a periodic array of pearls of the photoresponsive material along the capillary produced by the so-called Rayleigh-Plateau instability. The effect of the photoresponsive layer thickness and contact angle hysteresis of the solid-liquid interface were found to be important parameters in the photoinduced thermocapillary effect. Furthermore, a linear relationship between the optical intensity I(o) and droplet velocity v was found for a wide range of the former, allowing us to analyze the results and estimate response times for heat transfer using heat conduction theory.
热毛细流动已被证明是诱导和控制微通道中液滴和气泡运动的一种很好的替代方法。温度梯度通常通过在通道附近植入金属加热器或通过包含一层能够吸收辐射能量的光敏材料来建立。在这项工作中，我们表明，单个液滴可以通过涂有光响应复合材料（聚二甲基硅氧烷+碳纳米粉末）的毛细管泵送，并通过光纤用光源照射。用这种方法实现的最大液滴速度约为300μm/s，最大位移约为液滴长度的120%。已证明，无论是在毛细管表面有完整涂层，还是沿毛细管由所谓的瑞利-普拉托不稳定性产生的光响应材料的周期性珍珠阵列，涂层都具有发热能力。发现光响应层厚度和固液界面的接触角滞后效应是光致热毛细效应中的重要参数。此外，在很宽的光强I(o)范围内，发现光强与液滴速度v之间存在线性关系，这使我们能够分析结果并使用热传导理论估计热传递的响应时间。
