Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - VIBRATE (Self-sustaining vibration and mechanical resonance effects in stimuli responsive liquid crystal polymer coatings and membranes (Vibrate))

Teaser

The Vibrate project investigates plastic-based systems such as films and coatings that can be brought in a continuous motion or vibration. The motion can be fueled by light, as for instance generated by the sun, or by an electrical field. When a film or coating is in...

Summary

The Vibrate project investigates plastic-based systems such as films and coatings that can be brought in a continuous motion or vibration. The motion can be fueled by light, as for instance generated by the sun, or by an electrical field. When a film or coating is in continuous motion it can solve problems related to contamination. An example is the cleaning of solar panels from sand under dry or wet conditions. Solar farms in the desert benefit from high solar power but easily become contaminated during sand storms thus reducing the output power. A self-cleaning mechanism would solve this problem. One of the outcomes of the project is a coating of the which the surface deforms either by light or by a short electrical pulse. In order to realize this the Vibrate project focuses on liquid crystal polymers that can be brought out of their equilibrium and by utilizing feedback mechanisms can be brought into motion. This has resulted into a film that, when exposed to light, show a continuous wave deformation. It also has resulted in a coating that, when exposed to a moderate electrical field, form topographic deformations. On these deformation high-frequency oscillations take place that are effective in transporting material, such as sand, on top of the coating.

Work performed

Intermediate report Vibrate project period 01 October 2015 - 31 March 2018

The overall objective of the project is to develop new principles leading to systems or devices that provide oscillatory motion as well as coatings that provide vibrational motility in their surfaces. The motion can be fueled by a variety of energy sources, but in the Vibrate project the emphasis will be on light and electricity. In the Description of the Act three tasks were defined:
• Task a. The generation of responsive materials and processing them into devices
• Task b. Studies of the device responses to light and/or electrical fields
• Task c. Realization of self-sustained oscillation and resonance effects
In the reporting period we worked simultaneously on these tasks with our PhD and Postdoctoral students.


Task a. The generation of responsive materials and processing them into devices

Traditionally, and presently being studied by many research teams, photo-actuation of a liquid crystal network (LCN) or elastomer (LCE) is established by covalently incorporation of azobenzene molecules into the network. Besides the feature of photo-deformation, these molecules have a number of disadvantages such as a slow reversed reaction and they color the films and coatings yellow where the application mostly require colorless systems. Especially the slow back reaction is a problem in the creation of vibrational motion. For that reason, we have developed a new set of reactive azobenzene molecules, modified with push-pull moieties in the aromatic core, that have a fast back relaxation reaction (Nature 2017).

Next to the azobenzene compounds we developed a set of new trigger molecules that induce fast light response to our systems and have their absorption predominantly in the UV part of the spectrum and are therefore colorless. Among these are (hydroxyphenyl) benzotriazoles, benzophenones and dimethylaminobenzaldehyde (Advanced Materials 2017). Some of these molecules form the basis of photo-stabilizers in commercial plastics which provides the prospect of a better durability of our system during light-induced actuation. Based on these newly developed photo-triggers we made a set of oscillating devices. For instance, we demonstrated polymer beams that upon exposure with light oscillate following its natural frequency as determined by dimensions and modulus (Advanced Materials 2017). Based on hydrazone photo-triggers we produced a mill-like device of which the blades rotate under light exposure (Tetrahedron 2017). In addition, we produced perpetual morphing devices capable to create continuous transporting waves or to transport themselves when embedded in a frame (Nature 2017).

So far, the photo-sensitive molecules that trigger morphing of the films are sensitive for a single wavelength. We introduced a new set of pH sensitive azo dyes. Depending on the pH history the absorption can be positioned at a desired location. This enables localization of the trigger with respect to position and triggering wavelength, bringing origami type of morphing a step closer to realization (Angewandte Chemie 2017)

In order to bring the morphing elements closer to a device and application, we developed a method to produce an array of fibers on a glass substrate (responsive grass). A new liquid crystal monomer was synthesized to meet the rheological requirements to draw the fibers from printed droplets at the surface. Upon actuation the fibers were capable to transport species when immersed in a liquid with a viscosity equal to saliva thus mimicking transport in our respiratory system (Advanced Functional Materials 2016).


Task b. Studies of the device responses to light and/or electrical fields

Whereas in task a we mainly study free-standing LCN films or fibers, within this task we focus on films or coatings that firmly adhere to a solid substrate. Different principles are being applied to deform the surface. In a first approach we make use of changes of t

Final results

There are a number of project results within Vibrate that are unprecedented and beyond the state of the art of existing materials:

1. Within the project we developed organic coating materials that show topographical deformation when addressed by an in-plane electrical field. The electrodes are \'buried\' in the coating and cannot be touched by hand. Moreover the voltages are moderate. The topographies form when the field frequency is above a threshold value, which is an indication that we need to meet resonance conditions. What is especially spectacular, and definitely not predicted on forehand, that on top of the larger deformations there is a high frequency oscillation which is capable to transport material at the coating surface.
2. We can create coatings with a fingerprint surface structure with random hills and valleys of heights of the order of 100 nm. By applying an electrical field the surface topography can be inverted, i.e. hills become valleys and valleys become hills.
3. Thin films of liquid crystal polymer could be brought into a continuous wave by exposure with light from a LED light source. The splayed molecular organisation in the film is essential to control the deformation. What was unexpected is that the direction of the wave depends on the side at which the film is illuminated. When exposed at the side here the molecules are oriented planar, the wave moves away from the light source. When the light hits the side where the molecules are oriented perpendicular to the plane of the film, the wave moves towards the light source. When the wave-forming plastic film is mounted in a frame, it walks over the surface away from the light source or towards the light source, depending on the splay orientation.
4. Vibration and continuous motility was the main objective of the project. We developed a number of examples where that was achieved: oscillating polymer beams. polymer mill with light-driven blades, the plastic films with a continuous wave-like deformation, the surfaces with hierarchical motility.

In the second half of the project we anticipate to get a better understanding of the assumed resonance effects by laser speckle measurement and Fourier analysis of the high frequency oscillations. With respect to applications we will develop systems with controlled directional transport of particles at the surfaces. It is an overal objective to make the surface deformations higher such that new applications, such as haptics, might also become feasible. Thereto we will study hybrid systems of liquid crystal network and elastomeric polymers. Also in the second half of the project we will work further on the idea of self-pumping membranes which we could not realise so far because of various technological issues.