OFFPRINT Quantification of droplet deformation by electromagnetic trapping

A tightly focused laser beam exerting a trapping force on an object also exerts deformation forces on the object. If the object is relatively easy to deform, as is, e.g., a low surface tension droplet, the resulting deformation is easily detectable even at moderate laser powers. The observed deformation is analytically explained by a model, which quantitatively predicts the deformation of any micron-sized drop where the only restoring force is the surface tension. Theoretical tools are also provided to include the effect of elasticity of the shell and bulk of the trapped object, this being particularly important for deformations of cells. This deformation effect of electromagnetic radiation is important to consider while trapping soft materials and it can be used to determine physical characteristics of soft materials. Copyright c © EPLA, 2009 Introduction. – By tightly focusing a single laser beam an optical trap can be created. Optical traps are widely used to manipulate micro-meter–sized objects such as living cells and polystyrene beads [1] and even nanometer–sized metallic particles are easily manipulated [2,3]. Optical traps can also be used as force-measuring tools and have proven very successful in measuring corresponding values of forces and distances in single molecule assays, where the system of interest is conjugated to a bead, which serves as a handle for the optical routines [4]. It is a well-known physical fact that one cannot perform a measurement without influencing the measured system. This principle becomes more relevant as the size of the system decreases and is of outermost significance for quantum mechanics. For microand macroscopic systems this effect is often not relevant in practise, however, the effect is still there and it can even be advantageously utilized for instance as shown in this letter. Optical traps exert considerable forces on a trapped object. If the trapped material is deformable, the gradient in the light intensity will cause forces which tend to deform the object. This effect is beneficially utilized, e.g., in “optical stretcher” assays [5], which are typically used to deform living cells, and where the extend of deformation gives information about the viscoelastic properties of the deformed material. The light field from an optical trap has also been used to deform biological objects such as red blood cells [6,7] from their original biconcave disk shape (a)E-mail: [email protected] Fig. 1: Images of a low surface tension drop at different laser powers. From left to right: 40mW, 80mW, 126mW, and 252mW. into a more rod-like shape, interestingly, the optically induced deformation was shown to be dependent on the physiological state of the cell. Optically held beads have been used to stretch various types of cells [8] with the goal of elucidating chemomecanical pathways. Also, multiple optical traps have been used to perform optical sculpturing of emulsion droplets [9]. However, in order to truly take the effect into consideration in measurements involving tightly focused lasers, or to take advantage of it to study physical properties of soft materials, it is important to quantify and theoretically understand the effect. In this letter we use an optical trap based on a single infrared laser beam to deform low surface tension droplets. We show that an increase in laser power from zero to 200mW at the sample linearly decreases the droplet diameter by as much as 20% (as shown in fig. 1) and increases the droplet length along the direction of the propagating laser light by more than 50%. Our analytical model utilizes the surface tension, the indices of