Etchant preferred in gel form than in liquid form:

Generally, the etchant is supplied as an aqueous gel to allow precise placement over a specific area. These gels are often made by adding colloidal silica (the same fine particles used in microfilled composites) or polymer beads to the acid. Brushes are used to place the acidic gel, or the acid may be supplied in a disposable syringe from which it can be expressed onto enamel and dentin.  During placement, it is important to be aware of the risk for air bubbles that may be introduced at the interface. Regions with air pockets will not be etched. Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, 12th ed page no 262