Introduction
The
motivation behind the study is from dynamic responses from the skin of
cephalopods such as cuttlefish, octopus and squid that highly versatile in
projecting light patterns against its natural environment when receiving
external treats or transmitting signals for communication. Adaptive materials
able to spontaneously react to any forces applied and return to its original
structure when forces are removed. Recently, car bumpers can reshaped to its
original shape through heating the damaged part and thermo-sensitive polymers
used in toothbrushes to teach children to brush their teeth until the polymer
changes color.
Previously,
a group of researchers from Harvard University created a material with ability
to change its transparency when subjected to external force. This material is
fabricated by attaching different layer of materials together. This project
meant to exploit the adaptive capabilities of porous flexible films for tunable
transparency as an optical sensor. Porous film fabricated is made from
polydimethylsiloxane (PDMS). At various variables manipulations, different pore
characteristics are produced and investigated.
Scope
The
scope of the project involves fabrication process of porous PDMS films with
pore sizes ranging from micro to nanometer. The project context includes
fabrication of porous PDMS layer, imitation of previous work of adaptive
materials and testing of adaptive materials produced. Testing process includes
the transparency of the films when subjected to stretching, compressing, and
bending. Optical properties of materials measured using spectrophotometer.
Aims
This
project aims to fabricate a porous film made by a single material which is PDMS.
Previous work has proof the possibility of creating adaptive material that is
able to change it transparency as it is subjected to external change in
environment including stretching and compressing.
Background
& Previous Work
On April, 7th 2013 a team
researchers from Harvard University published a paper entitled “Adaptive
fluid-infused porous films with tunable transparency and wettability”. The material is made from layers of porous films filled with
infused liquid to allow it changes the properties when subjected to any stress
or strain. The highlight of the paper is how the materials changed it structure
which the surface area of the materials when it is stretched or compressed.
However, for this project the focus is on the transparency of the material
changed as the structure changed when it is stretched or compressed.
This material consists of four
layers. The bottom layer is an elastic PMDS film that attached to a Teflon
membrane using adhesive PDMS. Fluid lubricant is infused to the layer and
filled the pores of the porous layers and allows the material to react to its
surrounding. Any forces applied to this material will result in a change of the
structure and thereby affecting the properties of the materials. Fluid lubricant
helps in elastic deformation, fluid flow and solid-liquid interaction. When
forces applied are removed, materials structure recovers and turned to its
original structure. Selection process of the lubricant is depends on the
surface-energy that match solid matrix to ensure the layers stay attached to
the porous layers. Figure 1 shows the structure of the materials.
Figure 1: Structure of Adaptive
Liquid-Infused Materials
As shown in Figure 2, when material
is immersed into chloroform, its optical property change from translucent to
opaque resulted from absorption of liquid into the structure. The material is
turned back to its original state when it is dried in the oven. It is also has
the ability to heal from mechanism applied to it. Figure 4 shows the
self-healing mechanism of the material when it is pressed with finger leaving
the fingerprint on the surface. The fingerprint disappears when the material is
stretch back as the liquid in the structure return to its initial position.
Figure 2: Swelling and Drying Changes
Transparency of Material
Figure 3: Anti-fingerprint Mechanism of Material
In
order to create PDMS porous film, reference procedure is taken from paper
published by European Patent Application entitled Method of Fabricating a
Porous Elastomer. This paper explains method of fabricating porous elastomer
using water as liquid porogen. The process involved is homogenous emulsion of
liquid elastomer and liquid porogen which is water. From the published method,
size of pore can be varied by manipulating variables in the process.
Process
described in fabricating porous elastomer is by mixing liquid elastomer and
water in vacuum until homogenous emulsion is reached followed by curing the
mixture to reach polymerization. After that, water is removed from formed
polymer. Emulsion process is done using magnetic stirrer to ensure consistent
stirring process. Factors affecting the pore size are stirring time, stirring
speed and ratio of water to PDMS liquid. These three factors are discussed more
in other chapter.
Figure
4: Porous Elastomer Fabricated by Water-in-silicone Emulsion
Refabrication of Adaptive
Material
For purpose of refabrication of
adaptive material, PDMS layer with thickness less than 1mm is used due to it
higher flexibility and less thickness to allow more layers to be attached
together in later stage. As described in the paper, Teflon layer is bought from
Sterlitech Co. , PTFE Unlaminated Membrane Filter 0.2micron of pore size. Using
the PDMS film fabricated earlier, the procedure is continued by layering PDMS
elastomer and dried in the oven for 10 minutes before stacking Teflon layer on
the sticky oligomer. The sample is continued to be dried in the oven for next
10 minutes to allow Teflon layer properly attached to PDMS film. Finally,
Perfluropolyether fluid which is Dupont Krytox perfluropolyther 103 is drop
into the layer to finish the refabrication of adaptive material. The result of
the procedure is as follow:
Figure 5: Adaptive Material
Refabricated
The
material’s ability to change it transparency is tested. Material is placed on
top of the written letter that cannot be seen clearly and stretched by hand.
When stretched, the letters appears and can be seen with naked eyes. When the
material is released, it returns to its original length and covering the
letters again. Images below show the testing results:
Figure 15: Change of Transparency
of the Adaptive Material Fabricated
During the process of refabrication
the adaptive material, it is discovered that PDMS layer acts as backbone of the
Teflon layer as the oil will be passing through the pore. In order to hold the
oil in the structure and continuously fill the pore in Teflon layer, PDMS film
is used. As Teflon layer and PDMS film do not stick together easily, sticky
oligomer is made from PDMS liquid. This is due to its clear colour and easy to
control its change of phase from liquid to solid by altering the drying time.
Later in Chapter 2, it is discovered that sticky oligomer needs to be left in
the oven for 10 minutes to fabricate a good adaptive layer. Stacking top layer
too fast will cause the air trapped at the sticky oligomer layer and damage the
structure. Photo below shows the oil passing through Teflon pores:
Figure 16: Krytox Oil Passing
Through Pore in Teflon Layer and Drop on the Paper
