The Manipulation of Nematic Liquid Crystal Microdroplet in Water by Optical Tweezers
30 December 2025
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Written by:
Dr. Muhamad Safuan Mat Yeng
Senior Lecturer
Physics Department,
Faculty of Science and Mathematics,
Universiti Pendidikan Sultan Idris
Dr. Shahrul Kadri Ayop
Associate Professor
Physics Department,
Faculty of Science and Mathematics,
Universiti Pendidikan Sultan Idris
Introduction to Nematic Liquid Crystal Microdroplet
Liquid crystals are unique materials; they exhibit a liquid phase with a crystalline structure [1]. One of the most studied liquid crystals is a thermotropic liquid crystal [2]. This type of liquid crystal is sensitive to temperature change. For example, a thermotropic liquid crystal, 4-cyano-4’-pentylbiphenyl (5CB), has been widely used in applications such as optical trapping, LCD panels, sensing, and others [3]. At room temperature, 5CB exists in the nematic phase, which is a type of arrangement where the liquid crystal molecules lack positional order and align parallel to the director (n ̂) [3]. Fabrication of the nematic liquid crystal microdroplet required sonication of the liquid crystal at room temperature in water for 1 to 3 minutes [4]. This sonication resulted in a variety of nematic liquid crystal microdroplet suspensions of different sizes in water.
Figure 1: Nematic liquid crystal microdroplet suspension observed from optical microscopy.
In general, the nematic liquid crystal microdroplet adopts a bipolar internal configuration in which the liquid crystal molecules align parallel to the microdroplet surface. If a surfactant is added, the bipolar internal configuration transforms to a radial configuration, in which the liquid crystal molecules align perpendicular to the microdroplet surface [4]. These two internal configurations form a characteristic pattern that distinguishes how the liquid crystal molecules are arranged in the nematic phase within the microdroplet. Figure 1 shows an optical image of the nematic liquid crystal microdroplet suspension in water.
Optical Manipulation of Nematic Liquid Crystal Microdroplet
Nematic liquid crystal in the form of a microdroplet can be optically manipulated in water by optical tweezers. The optical force of the optical tweezers can trap and spatially confine the nematic liquid crystal microdroplet [3]. The possible reason is that the nematic liquid crystal microdroplet has an effective refractive index higher than that of the surrounding water; hence, the optical force pulls it toward the laser focus. Since the optical force increased with the trapped material's refractive index, the liquid crystal microdroplet with a refractive index higher than that of water was attracted to the laser focus [5]. Figure 2 shows the image sequence of a nematic liquid crystal microdroplet of 0.7 μm size optically trapped into the laser spot. The laser power used is 0.2 MW/cm². The free-flowing nematic liquid crystal microdroplet in Figure 2: (a)-(b) is trapped by the optical tweezer laser spot (indicated by red dashed circle) as shown in Figure: 2 (c)-(d). Figure 2: (d) shows the optically trapped nematic liquid microdroplet (bright spot) viewed under a crossed polarized image to verify its internal configuration.
Figure 2: (a) – (b); The nematic liquid crystal microdroplet is moving freely. (c)-(d); The nematic liquid crystal microdroplet trapped at the center of the laser spot.
[The red dashed circle indicates the optical trap]
Other than optically trapping a nematic liquid crystal microdroplet, the optical tweezers can also manipulate the arrangement of the liquid crystal molecules inside the microdroplet. Figure 3 shows the image sequence of manipulation of 2.0 μm nematic liquid crystal microdroplet (a)-(d) under optical microscopy and (a')-(d') corresponding image under crossed polarized view. The red arrow indicates the direction of the linearly polarized laser. As the laser polarization angle is rotated clockwise from 0 to 90, the optical microscopy image in Figure 3 (a)-(d) cannot clearly show the optical texture change, indicating the internal configuration change. However, the cross-polarized view images in Figure 3 (a')-(d') show the optical texture (bright spot) changes with the increasing angle of laser polarization. This is evidence that the laser polarization state affects the internal alignment configuration of liquid crystal molecules within the microdroplet.
Figure 3: The manipulation of the nematic liquid crystal internal configuration by a linearly polarized laser beam.
In a cross-polarized view, the nematic liquid crystal microdroplet appears as a bright spot due to light scattering within it [6]. The optical texture changes with laser polarization direction because of its birefringence. The birefringence is correlated with the laser's refractive index and optical force [7-8]. Hence, this optical force aligns the nematic liquid crystal molecules with the laser polarization direction.
As a conclusion, the nematic liquid crystal in the form of a microdroplet can be optically manipulated using a laser. Manipulating the nematic liquid crystal microdroplet in water is significant for prospective applications such as probing, microactuation, inducing mechanical rotation, and others. Researchers are continually working to understand how the behavior of the soft microdroplet affects optical force quantification, as this is very challenging.
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