Universal Plasma Jet for Droplet Manipulation on a PDMS Surface towards Wall-Less Scaffolds

Abstract

Droplet manipulation is important in the fields of engineering, biology, chemistry, and medicine. Many techniques, such as electrowetting and magnetic actuation, have been developed for droplet manipulation. However, the fabrication of the manipulation platform often takes a long time and requires well-trained skills. Here we proposed a novel method that can directly generate and manipulate droplets on a polymeric surface using a universal plasma jet. One of its greatest advantages is that the jet can tremendously reduce the time for the platform fabrication while it can still perform stable droplet manipulation with controllable droplet size and motion. There are two steps for the proposed method. First, the universal plasma jet is set in plasma mode for modifying the manipulation path for droplets. Second, the jet is switched to air-jet mode for droplet generation and manipulation. The jetted air separates and pushes droplets along the plasma-treated path for droplet generation and manipulation. According to the experimental results, the size of the droplet can be controlled by the treatment time in the first step, i.e., a shorter treatment time of plasma results in a smaller size of the droplet, and vice versa. The largest and the smallest sizes of the generated droplets in the results are about 6 L and 0.1 L, respectively. Infrared spectra of absorption on the PDMS surfaces with and without the plasma treatment are investigated by Fourier-transform infrared spectroscopy. Tests of generating and mixing two droplets on a PDMS surface are successfully achieved. The aging effect of plasma treatment for the proposed method is also discussed. The proposed method provides a simple, fast, and low-cost way to generate and manipulate droplets on a polymeric surface. The method is expected to be applied to droplet-based cell culture by manipulating droplets encapsulating living cells and towards wall-less scaffolds on a polymeric surface.

Impacting Droplet Can Mitigate Dust from PDMS Micro-Post Array Surfaces

“…1 for droplet 0.5 for interface 0 for air (5) It is worth mentioning that the initial condition for the droplet and air temperature settings were based on the experimental conditions in the lab environment. In addition, the droplet initial velocity of 0.536 m/s corresponds to the experimental droplet impact height, which was measured as 14.64 mm.…”

“…Furthermore, the convective boundary condition is applied at the droplet-air interface, while thermal insulation is imposed at all exterior boundaries (i.e., boundaries (1)-( 3)). The substrate surface (i.e., boundary (5)) is considered to be a wetted wall with predefined contact angle and slip velocity. Boundary (4) is also considered as a slip boundary.…”

“…For numerical calculations, a suitable choice is β = h 5 , where h is the edge length of the mesh element. It is worth mentioning that the micro-post array surfaces are hydrophobized and to account for the hydrophobic surface, the slip condition is considered at the interface between the droplet liquid and micro-post array surface, i.e., a hydrophobic wetting state is adopted at the boundary (5). In addition, the pinning force (F ad ) is added to account for the droplet adhesion over the three-phase-contact-line under the surface tension.…”

“…Boundaries ( 1)-( 3) are the pressure outlets. In addition, experimentally obtained adhesion/frictional forces are defined at the boundary (5) from which the droplet motion is inhibited by the resistive interfacial forces. Hence, the adopted boundary conditions are expressed as:…”

“…The impact of liquid droplets on hydrophobic surfaces has various applications, some of which include biomedicine [1], combustion [2], forming and coating [3], and self-cleaning [4]. Although various mechanisms are adopted for droplet mobility on surfaces such as polydimethylsiloxane (PDMS) surface [5], the surface texture [6] and droplet fluid properties [7] remain the important properties assessing the droplet behavior. The topology of hydrophobic surfaces has hierarchically distributed micro/nanopillars and cavities and depending on the texture characteristics, such as texture profile, texture spacing, and pillar heights, the contact angle of droplet changes [8].…”