Make+a+Contact+Angle+on+Surface

Making contact angles is an important factor in knowing whether or not your monolayer even exists on your surface. Sometime, the solutions won't form on top of the whole silicon surface, and occasionally not at all. Therefore, making contact angles will help you decide whether or not a monolayer has formed. Why should we make contact angles? All solutions that act as monolayers are either hydrophobic or hydrophilic. A **hydrophobic** monolayer will have a water droplet with a contact angle greater than 90 degrees. A **hydrophilic** contact angle will be less than 90 degrees. A **hydrophobic** monolayer means that the water will be "afraid" of the surface and it will bead up on the surface. A **hydrophilic** monolayer will have the droplet spread out over the surface as the water is attracted to it.

Both PEG and APTES solutions should be hydrophilic. APTES contact angles should be between 54 degrees and 68 degrees, while PEG solution contact angles should be between 26 degrees and 37 degrees. Only OTS solution is hydrophobic with contact angles between 110 degrees and 120 degrees. However, OTS solution can't be used in this project as the DNA does not form properly on it, so it is not necessary to know it. If you do not get these measurements (more or less), you are doing something wrong.

Here is an example of taking contact angles of a drop of water on a surface. This image is from [|www.nature.com].



This rough example of a contact angle device has been taken from [|www.chanceint.com].



1.) Place the sample on the platform of the contact angle machine. Put a small 18 mega ohm water droplet on top of your sample (silicon chip with monolayer on top) using the pipette connected to the movable column. Leave the pipette in the droplet.

2.) Peer through the magnifying glass (not seen in this image) to find a protractor zoomed in on your water droplet. Using a knob on your right, manipulate the cross hairs of the protractor to measure the angle of your droplet on the monolayer.

3.) Remove the pippete and write down the measurement that the cross hair makes on the protractor that you see through the magnifying glass. Do this four times in four different spots of your sample to make sure that the angles are right.

Here is an image I made of how contact angles work. This image is of a hydrophilic sample.



Next Milestone - Deposit the DNA