Thermocapillary dynamics of liquid plugs in a capillary tube

Transporting miniscule liquid volumes is essential for operation of droplet-based microfluidic devices and development of effective techniques for droplet manipulation are needed'. The liquid droplets are generally actuated in microchannel (1-D platforms) or on a surface (2D) and is generally achieved by various techniques such as use of chemical gradients, geometrical morphology, pressure gradients, electric, magnetic fields etc. Thermal gradients can also be employed to induce the motion of the droplet where the temperature dependent surface tension generates the thermocapillary force which, propels the liquid droplet. In the present work, experimental and numerical investigations are focused on thermocapillary motion of liquid plug inside a capillary tube (1-D platform). When a small amount of liquid is present inside a capillary tube, a liquid plug with two menisci is formed. The heating of one end of the capillary tube results in a surface tension reduction at the meniscus near the heater. This induces a net capillary pressure gradient which propels the liquid plug away from the heater'. Experiments are performed studying the motion of silicone oil liquid plug inside a soda lime glass capillary tube. The capillary tube is heated using a nichrome wire heater which maintains constant heat flux at one end. The temporal evolution of liquid plug position and temperature are captured simultaneously using a CMOS and IR camera. The effect of oil viscosity, liquid plug length, capillary tube diameter and heat flux on the velocity of the liquid plug are studied. The change in migration dynamics due to the presence of another liquid plug is also investigated. It is observed that the increase in oil viscosity diminishes the liquid plug velocity whereas the increase of liquid plug length, capillary tube diameter and heat flux are observed to augment the maximum velocity of the plug. Further-, the transient evolution of velocity of the liquid plug is observed to be considerably effected with change of each parameter. In the experiments, evolution of the thermal field is transient in nature and the effect of parameters influencing the liquid plug velocities cannot captured to full extent It is difficult to design a setup to study the motion of liquid plug without these transient effects. Instead, a comprehensive numerical study is performed where liquid plug motion is studied without the effect of thermal transient. OpenFOAM based in-house two phase solver with iso-surface based interface reconstruction is used for the present numerical study. The solver was validated to simulate the thermocapillary phenomenon at the free surface and additional validation studies are performed with the current experiments to confirm the ability of the solver to capture of contact line dynamics during thermocapillary migration. Numerical studies are performed to study the change of velocity of migration with change of the liquid plug properties, liquid plug dimensions and capillary tube diameter. Additionally, the effect of thermal conductivity of the capillary tube on evolution of velocity is also investigated which is difficult to delineate experimentally due to non-transparency of high thermal conductivity capillary tubes. The present work reveals the importance of the thermocapillary farces on liquid plugs which are of paramount importance in systems involving multiple phases such as heat pipes, flow boiling equipment etc. (author)