Switching between positive and negative movement near an air/water interface through lateral laser illumination

Abstract

The directional movement of an aqueous solution containing gold nanoparticles under laser irradiation (532 nm) through the vicinity of an air/water interface is reported. It is shown that unidirectional flow is generated along the path of light-travel when a laser is irradiated parallel to the interface. On the contrary, liquid flow toward the light source is caused when the incident angle of the laser is tuned so as to make the total reflection at an air/water interface. Such switching of the flow direction, positive or negative to the light propagation, was applied to the movement of mm-sized floating object, revealing that repetitive back and forth motion of the solid object is generated. This directional switching of photo-induced movement is interpreted by taking into account the flow caused by the spatial gradient of the surface tension due to the thermal effect of the incident laser. A numerical model with a Navier–Stokes-type equation reproduces the essential aspects of the switching of the direction observed in the present study.

Relaxation to steady states of a binary liquid mixture around an optically heated colloid

“…Then, it is of immediate interest to know how quickly the medium near the particle changes its structure after switching on illumination, compared with the speed of return to the equilibrium state after switching it off and how this can be controlled. Understanding relaxations of binary liquid mixtures in presence of temperature gradients is also of * maciolek@is.mpg.de greatest relevance for applications, such as in optical fluid pumps [29], phase separation in microfluidic cavities [30], and phoretic motion of colloids in liquid mixtures [31].…”

Self‐Sustained Marangoni Flows Driven by Chemical Reactions**

“…Light has also been investigated as an external stimulus to remotely control these interfacial flows. While it has been used directly to locally heat an interface, [32] recent works have shown its potential when combined with photo-switchable surfactants [33][34][35][36][37] for the manipulation of μm-range objects. In that case, light intensity and surfactant concentration were used to regulate the flow velocity.…”

Self-sustained Marangoni Flows Driven by Chemical Reactions

“…Light has also been investigated as an external stimulus to remotely control these interfacial flows. While it has been used directly to locally heat an interface 31 , recent works have shown its potential when combined with photo-switchable surfactants 32,33 for the manipulation of μmrange objects. In that case, light intensity and surfactant concentration were used to regulate the flow velocity [34][35][36] .…”