Noncontact Bubble Manipulation in Microchannel by Using Photothermal Marangoni Effect

A novel method of noncontact bubble manipulation by optically-induced local surface tension gradient is described in this paper. In microfluidic devices, the effects of interfacial phenomena become dominant with decreasing of a length scale. An unexpected adhesion of a bubble on the channel wall is a serious problem which can cause the large pressure loss and the deterioration of the device. Thus, the removal or manipulation technique of the bubble is strongly required. In this study we controlled the thermocapillary force around the bubble by means of optical technique. The purpose of this study is the verification of the optical manipulation method of bubble. Particularly, the detail of migration process including the effect of bubble size, fluid viscosity and optical power is discussed. The manipulation experiments were conducted for the bubble with the diameter of 40 to 140 μm in a microchannel filled with silicone oil. An FEP (fluorinated ethylene propylene copolymer) tube with the inner diameter of 200 μm was used as the microchannel. The optical system for the heating is composed of a scanning setup and a compact laser diode. In this technique, two types of motion for the bubble transport are possible. One motion is the detachment of the bubble from the channel wall. When a laser beam is irradiated into the liquid in the vicinity of the bubble attached to the wall, the Marangoni convection is induced and the difference of pressure is generated around the bubble. The bubble is detached from the wall when the pressure difference overcomes the anchoring force between the bubble and the wall. Then, the bubble detached from the wall is suspended in the liquid at the balanced position between the thermocapillary and buoyancy force. This position can be controlled by adjusting the laser power. The other motion is the manipulation of the bubble along the channel. When the focal spot is scanned along the channel, it is possible to manipulate the bubble as if the bubble follows the light. In addition, the minimum optical power necessary to transport the bubble along the microchannel was measured. The minimum optical power strongly depends on bubble size, liquid viscosity, and scanning speed. These results show the relationship between the driving force induced by photothermal Marangoni effect and the resistance force related with the viscosity and the scanning speed.