Modelling thermocapillary migration of a microfluidic droplet on a solid surface
Modelling thermocapillary migration of a microfluidic droplet on a solid surface
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DOI：
10.1016/j.jcp.2014.09.015
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发表时间：
2015
期刊：
影响因子：
--
通讯作者：
Haihu Liu;Yonghao Zhang
中科院分区：
文献类型：
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作者：
Haihu Liu;Yonghao Zhang
建立了一个多相晶格玻尔兹曼模型来模拟具有流体表面相互作用的非混相热毛细管流动。该模型通过引入基于连续表面力概念的体力项，纳入界面张力和Marangoni应力，并采用拉脱维亚- kokko和罗斯曼提出的重着色算法实现相分离。在固体表面，流体-表面相互作用由部分润湿边界条件建模，该条件使用几何公式指定接触角，并使用保色边界闭合方案来提高数值精度并抑制接触线上的伪速度。通过被动标量法求解另一个对流扩散方程得到温度场，并通过状态方程与流体动力方程耦合。首先通过静态接触角和恒定界面张力条件下毛细管动态侵入过程的仿真验证了该模型的有效性。然后用它来模拟微流控液滴在均匀温度梯度下在水平固体表面上的热毛细迁移。我们首次在数值上证明了液滴的运动经历了两种不同的状态，这取决于表面的润湿性：液滴向亲水表面的较冷区域迁移，而在疏水表面则相反。降低粘度比可以增强热毛细涡的强度，导致迁移速度的增加。接触角迟滞，即前后接触角之差，与接触角和粘度比无关，始终为正。接触角滞后量和偏移速度随接触角的增大先减小后增大，最小值出现在接触角为90°时。
A multiphase lattice Boltzmann model is developed to simulate immiscible thermocapillary flows with the presence of fluid–surface interactions. In this model, interfacial tension force and Marangoni stress are included by introducing a body force term based on the concept of continuum surface force, and phase segregation is achieved using the recolouring algorithm proposed by Latva-Kokko and Rothman. At a solid surface, fluid–surface interactions are modelled by a partial wetting boundary condition that uses a geometric formulation to specify the contact angle, and a colour-conserving boundary closure scheme to improve the numerical accuracy and suppress spurious velocities at the contact line. An additional convection–diffusion equation is solved by the passive scalar approach to obtain the temperature field, which is coupled to the hydrodynamic equations through an equation of state. This model is first validated by simulations of static contact angle and dynamic capillary intrusion process when a constant interfacial tension is considered. It is then used to simulate the thermocapillary migration of a microfluidic droplet on a horizontal solid surface subject to a uniform temperature gradient. We for the first time demonstrate numerically that the droplet motion undergoes two different states depending on the surface wettability: the droplet migrates towards the cooler regions on hydrophilic surfaces but reverses on hydrophobic surfaces. Decreasing the viscosity ratio can enhance the intensity of thermocapillary vortices, leading to an increase in migration velocity. The contact angle hysteresis, i.e., the difference between the advancing and receding contact angles, is always positive regardless of the contact angle and viscosity ratio. The contact angle hysteresis and the migration velocity both first decrease and then increase with the contact angle, and their minimum values occur at the contact angle of 90 degrees.