The two worked together in the university's urology department, sharing a small laboratory on the university's Parnassus campus. Liu was arrested Monday after he allegedly confessed to Cao, who then alerted police.
When confronted by authorities, Liu admitted to police that he twice spiked Cao's drink, first on Oct. 29 and then again about a week later, according to university police captain Paul Berlin.
"On Nov. 7, we interviewed Liu and he confessed," Berlin said. "Two search warrants were executed at his home and lab, and evidence was recovered in both locations."
Investigators said Liu slipped a buffering agent, a chemical used in experiments to stabilize compounds, into Cao's drink. Cao reportedly noticed that her water had turned blue, but drank it anyway. She reported the suspicious drink -- which she consumed at the lab -- to police two weeks before Liu confessed to allegedly poisoning it.
"She noticed it was discolored, but drank it anyway," Berlin said. "It doesn't seem to have made her sick in the short term but we don't know the effects this kind of chemical might have on someone in the long-term. We really don't know much about the chemical's effect on people."
"She never seemed to show any symptoms but was taken to the hospital to be examined and has since been released," he said.
Liu is a Chinese national in the United States on a work visa. He is being held in the San Francisco County jail on immigrant hold. No bail has been set and he has yet to obtain legal counsel. He is expected to be arraigned today or Thursday.
Authorities said Liu would not give a reason for why he allegedly poisoned Cao's drink aside from that he was "stressed out."
"He won't give us a motive, but just keeps saying he's 'stressed out,' over and over," Berlin said. "We can speculate why he did it: family issues, work issues, financial issues, and personal issues.
"We know they were not linked romantically. They worked closely in a small 800-square-foot lab, and you can speculate that there was a fair amount of competition in there," he said.