Several forces cause the liquid to move upwards in a straw. Among them are capillary action, viscosity, suction, adhesion, cohesion, and atmospheric pressure.
We will start with atmospheric pressure. Fluids move from areas of higher pressure to areas of lower pressure. When sucking on a straw, air is removed from inside the straw. The air pressure outside the straw is greater than the air pressure inside the straw; therefore, it pushes down on the surface of your drink. This push forces the drink up through the straw and into your mouth.
The second force is capillary action. Capillary action is when liquids climb the walls of a tube or pull away from them. It occurs due to cohesion and adhesion. Cohesion is the forces of attraction between the molecules of the liquid. Molecules of a liquid are either charged positive or negative in accordance to the atoms the liquid is made of. For instance, water is made of hydrogen and oxygen. Oxygen is partly negatively charged and partially positively charged. This charging keeps the water molecules held together by the hydrogen bond. In all liquids, some type of bonding will be found. While the liquid is climbing up, cohesion keeps the liquid together and intact. Adhesion is the attracting of liquid molecules to each other. In this situation, it is the surface of the water that climbs. Adhesion helps move forward against gravitational forces. Thus, capillary action uses adhesion and cohesion to help the liquid cling to the sides of the straw.
Suction also plays a significant part when liquids are moving upward. When you suck on a straw, the straw creates a vacuum (low pressure) on one side of the straw while atmospheric pressure and water column pressure are pressuring on the waterside. The different pressure causes the water to rise into your mouth.
Viscosity is very important in the rising of liquids through a straw. If the liquid's viscosity is greater, more force is needed to maintain fluid flow through the straw. More suction is needed. If the liquid has a very high viscosity, it may not have the ability to flow through the straw.
From all the forces causing the liquid to move upward, I would have to say that capillary action is the most significant. Without this force, the liquid would not be able to cling to the sides of the straw, therefore not being able to reach the mouth of the person.
