Electrons flow from the aluminum to the silver amalgam filling. Aluminum foil will begin to decompose in the presence of many other acidic substances in a process called oxidation. Acids like to oxidize obliging metals. Tell any cooks you know to never wrap a meatloaf glazed with ketchup or tomato sauce in aluminum foil for storage. After several hours the result of this contact is a grayish-black disgusting mush of aluminum oxide.
Chemistry solves that problem, too.
Giant Bubbles Kids, you can make gigantic soap bubbles with a mixture of liquid soap or detergent, glycerin, and water. The glycerin is the secret ingredient that adds strength to the bubble solution. Pour into a large tray or cookie sheet. Dip the wire circle into the bubble solution and bring it out at an angle so that a film of solution fills the inside of the circle. Now sweep the wire through the air to form a large bubble.