Optical Manipulation of Microscale Fluid Flow

A novel optical method is used both to probe and to control dynamics in experiments on the spreading of microscale liquid films over solid substrates. The flow is manipulated by thermally induced surface-tension gradients that are regulated by controlling the absorption of light in the substrate. This approach permits, for the first time, the measurement of the dispersion relation for the well-known contact line instability; the measurements are compared with theoretical predictions from the slip model for spreading films. The experiments also demonstrate the use of feedback control to suppress instability. These results show that optical control can provide dynamically reconfigurable manipulations of fluid flow, thereby suggesting a general approach for constructing reprogrammable microfluidic devices.

Figure 1

Schematic illustration of microflow that is optically driven via the thermocapillary effect. An intensity-modulated beam from a light source illuminates a substrate that supports a tiny quantity of liquid at one end. Temperature variations arise from light absorption and induce surface tension gradients that drive the flow from the brighter (hotter) to darker (cooler) regions on the substrate. In all experiments, a uniform intensity gradient is applied to produce a temperature gradient of $30°\mathrm{C}$ per centimeter that drives the flow. In addition, suitable spatial and temporal perturbations of the intensity are employed to probe and to control pattern dynamics.

Figure 2

Experimental measurements (circles) are compared with theoretical predictions (lines) of the slip model for the spreading of thin liquid film on a solid substrate. Error bars on the experimental data are less than the symbol size. (a) The capillary ridge behind the contact line (located at the origin) is the dominant topographical feature of the streamwise profile of an unstable spreading film. (b) The dispersion relation for a spreading film with a profile shown in (a).

Figure 3

Video images under monochromatic illumination show the optical selection and control of thin film flow patterns on horizontal substrates. For each image in (a), (b), and (c), the film pattern is shown at two different times. The lower third of each image displays a thin film whose contact line is initially straight. The film is perturbed with a sinusoidally varying light intensity applied just behind the contact line; this perturbation induces a transverse modulation in the thickness, as indicated by interference fringes. The upper two-thirds of (a)–(c) show the subsequent formation of rivulets in a nonlinear regime at later times after the perturbations have been turned off. In these cases, the intensity variations displayed by the images arise solely from interference fringes, which can be used to measure the film’s topography. The pattern wave numbers are (a) $2.60{\mathrm{m}\mathrm{m}}^{-1}$, (b) $4.52{\mathrm{m}\mathrm{m}}^{-1}$, and (c) $6.17{\mathrm{m}\mathrm{m}}^{-1}$. In (d), the contact line instability is suppressed on the right half of a spreading front by feedback via illumination variations applied just behind the contact line. The left half of the (unstable) front evolves in the presence of a uniform constant thermal gradient to form rivulets.