Browsing by Author "Al-Azawi, Anas"

Now showing 1 - 6 of 6

Controllable fabrication of periodic arrays of magnetically actuated polymeric microstructures
(2015-02-04) Tupasela, Topi
Perustieteiden korkeakoulu | Bachelor's thesis
Friction and Wetting Transitions of Magnetic Droplets on Micropillared Superhydrophobic Surfaces
(2017-10-11) Al-Azawi, Anas; Latikka, Mika; Jokinen, Ville; Franssila, Sami; Ras, Robin H.A.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Reliable characterization of wetting properties is essential for the development and optimization of superhydrophobic surfaces. Here, the dynamics of superhydrophobicity is studied including droplet friction and wetting transitions by using droplet oscillations on micropillared surfaces. Analyzing droplet oscillations by high-speed camera makes it possible to obtain energy dissipation parameters such as contact angle hysteresis force and viscous damping coefficients, which indicate pinning and viscous losses, respectively. It is shown that the dissipative forces increase with increasing solid fraction and magnetic force. For 10 µm diameter pillars, the solid fraction range within which droplet oscillations are possible is between 0.97% and 2.18%. Beyond the upper limit, the oscillations become heavily damped due to high friction force. Below the lower limit, the droplet is no longer supported by the pillar tops and undergoes a Cassie–Wenzel transition. This transition is found to occur at lower pressure for a moving droplet than for a static droplet. The findings can help to optimize micropillared surfaces for low-friction droplet transport.
Programmable and Responsive Superhydrophobic Surfaces
(2021) Al-Azawi, Anas
School of Science | Doctoral dissertation (article-based)
Progress in the field of superhydrophobic surfaces requires precise characterization techniques and synthesis of surfaces that exhibit robust non-wettability. In this thesis, microfabrication techniques are used to produce static as well as bioinspired responsive superhydrophobic structures. In addition, transverse droplet oscillations are implemented to accurately evaluate superhydrophobicity of micropillared surfaces. Variations in surface properties that influence the degree of superhydrophobicity were successfully probed by relating friction and viscous dissipation of dynamic droplets to pattern density and chemical coating. Additionally, controlling the normal force exerted on the water-based ferrofluid droplet allows the measurement of impalement pressure necessary to induce wetting transition for a droplet in motion. A new fabrication process was introduced for rapid prototyping of cilia-inspired magnetic micropillars. The fabricated array of sub-10 µm diameter pillars are based on polydimethylsiloxane (PDMS) loaded with carbonyl iron particles (CIP). Lubricating in silicone oil allowed controlled droplet motion at the sub-mm scale facilitated by fast actuation and superhydrophobicity of the oil infused PDMS magnetic micropillars. Lack of mechanical stability due to flexibility of the high aspect ratio PDMS micropillars restricted the application of the array to liquid media only. Thiol-ene based magnetic micropillar arrays were introduced to address the stability issue of the high-aspect ratio micropillars. The remarkable properties of thiol-ene including tunability of surface and mechanical properties allowed topography modification of the magnetic micropillars using photo-induced thiol-ene click coupling. Decorating the surface of the high aspect ratio thiol-ene micropillars with polyvinyltrimethoxysilane (PVTMS) colloidal micro- and nanoparticles enhanced the mechanical stability of the flexible micropillars without compromising powerful bending actuation. This allowed actuation of the micropillar arrays in air as well as in liquid media. The magnetic micropillars were rendered superhydrophobic by grafting hydrophobic self-assemnled monolayer onto the PVTMS micro- and nanoparticles that are covalently bonded to the surface. This enabled directed water droplet motion by repetitive bending and recovery of the micropillars. Combining mechanical stability with robust superhydrophobicity can lead to numerous practical applications of cilia-inspired thiol-ene magnetic micropillars.
Slippery and magnetically responsive micropillared surfaces for manipulation of droplets and beads
(2020-08-01) Al-Azawi, Anas; Horenz, Christoph; Tupasela, Topi; Ikkala, Olli; Jokinen, Ville; Franssila, Sami; Ras, Robin H. A.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Stimuli-responsive surfaces are of practical importance for applications ranging from enhanced mixing of reagents in lab-on-a-chip systems until probing cellular traction forces. Non-destructive reversible bending of cilia-inspired magnetic pillars can be used for controlled transportation of non-magnetic objects and bio-inspired sensing. Magnetic actuation of micropillars suspended in liquids allows controlled mixing, propelling, and stirring of fluids as well as droplet manipulation, which are important for various applications including generation of cell spheroids and droplet coalescence in microfluidic systems. In order to expand their practical applications, fabrication processes capable of rapid prototyping have to be developed. Inspired by biological cilia and their functionalities, actuating hairy surfaces are herein fabricated and implemented to manipulate both microbeads and droplets. The artificial cilia are based on microscale magnetic pillar arrays made of flexible polydimethylsiloxane functionalized with magnetic microparticles. The arrays are fabricated by a new method using patterned molds that relies on cryogenic separation to produce transparent cilia-inspired arrays without requiring manual interference to clean the templates during the process. Magnetic actuation of the pillar arrays is demonstrated in isopropanol and silicone oil. Filling with oil yields magnetically responsive slippery lubricated surfaces allowing directional motion of droplets by repetitive bending and recovery of the flexible magnetic pillars. The achieved structures allow manipulation of microbeads and droplets which is uncommon even at the sub-mm scale; directional motion is demonstrated for 250 μm-550 μm sized droplets. Droplet transportation is facilitated by extremely low hysteresis and a high degree of omnidirectional bending of the pillar array.
Superhydrophobic Silicon Micropillar Arrays
(2018-04-27) Mäenpää, Roni
Perustieteiden korkeakoulu | Bachelor's thesis
Tunable and Magnetic Thiol-ene Micropillar Arrays
(2020-01) Al-Azawi, Anas; Cenev, Zoran; Tupasela, Topi; Peng, Bo; Ikkala, Olli; Zhou, Quan; Jokinen, Ville; Franssila, Sami; Ras, Robin
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Tunable and responsive surfaces offer routes to multiple functionalities ranging from superhydrophobic surfaces to controlled adhesion. Inspired by cilia structure in the respiratory pathway, magnetically responsive periodic arrays of flexible and magnetic thiol–ene micropillars are fabricated. Omnidirectional collective bending of the pillar array in magnetic field is shown. Local non‐contact actuation of a single pillar is achieved using an electromagnetic needle to probe the responsiveness and the elastic properties of the pillars by comparing the effect of thiol–ene crosslinking density to pillar bending. The suitable thiol–ene components for flexible and stiff magnetic micropillars and the workable range of thiol‐to‐allyl ratio are identified. The wettability of the magnetic pillars can be tailored by chemical and topography modification of the pillar surface. Low‐surface‐energy self‐assembled monolayers are grafted by UV‐assisted surface activation, which is also used for surface topography modificationby covalent bonding of micro‐ and nanoparticles to the pillar surface. The modified thiol–ene micopillars are resistant to capillarity‐driven collapse and they exhibit low contact angle hysteresis, allowing water droplet motion driven by repeated bending and recovery of the magnetic pillars in an external magnetic field. Transport of polyethylene microspheres is also demonstrated.